diff --git a/.gitignore b/.gitignore
index b9ae205..8842df9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -55,3 +55,6 @@ repl-temp-*
 # Dependency directories
 /ui/node_modules
 /ui/.parcel-cache
+
+# Test lua files
+/inbucket.lua
diff --git a/pkg/extension/broker.go b/pkg/extension/broker.go
index 93caad3..fa711eb 100644
--- a/pkg/extension/broker.go
+++ b/pkg/extension/broker.go
@@ -1,6 +1,10 @@
 package extension
 
-import "sync"
+import (
+	"errors"
+	"sync"
+	"time"
+)
 
 // EventBroker maintains a list of listeners interested in a specific type
 // of event.
@@ -55,3 +59,38 @@ func (eb *EventBroker[E, R]) lockedRemoveListener(name string) {
 		}
 	}
 }
+
+// AsyncTestListener returns a func that will wait for an event and return it, or timeout
+// with an error.
+func (eb *EventBroker[E, R]) AsyncTestListener(capacity int) func() (*E, error) {
+	const name = "asyncTestListener"
+
+	// Send event down channel.
+	events := make(chan E, capacity)
+	eb.AddListener(name,
+		func(msg E) *R {
+			events <- msg
+			return nil
+		})
+
+	count := 0
+
+	return func() (*E, error) {
+		count++
+
+		defer func() {
+			if count >= capacity {
+				eb.RemoveListener(name)
+				close(events)
+			}
+		}()
+
+		select {
+		case event := <-events:
+			return &event, nil
+
+		case <-time.After(time.Second * 2):
+			return nil, errors.New("Timeout waiting for event")
+		}
+	}
+}
diff --git a/pkg/extension/host.go b/pkg/extension/host.go
index 5d3ab3e..bef9846 100644
--- a/pkg/extension/host.go
+++ b/pkg/extension/host.go
@@ -20,8 +20,9 @@ type Host struct {
 // processed asynchronously with respect to the rest of Inbuckets operation.  However, an event
 // listener will not be called until the one before it complets.
 type Events struct {
-	AfterMessageStored EventBroker[event.MessageMetadata, Void]
-	BeforeMailAccepted EventBroker[event.AddressParts, bool]
+	AfterMessageDeleted EventBroker[event.MessageMetadata, Void]
+	AfterMessageStored  EventBroker[event.MessageMetadata, Void]
+	BeforeMailAccepted  EventBroker[event.AddressParts, bool]
 }
 
 // Void indicates the event emitter will ignore any value returned by listeners.
diff --git a/pkg/message/manager.go b/pkg/message/manager.go
index b62f268..bd89d79 100644
--- a/pkg/message/manager.go
+++ b/pkg/message/manager.go
@@ -98,7 +98,7 @@ func (s *StoreManager) GetMetadata(mailbox string) ([]*event.MessageMetadata, er
 	}
 	metas := make([]*event.MessageMetadata, len(messages))
 	for i, sm := range messages {
-		metas[i] = makeMetadata(sm)
+		metas[i] = MakeMetadata(sm)
 	}
 	return metas, nil
 }
@@ -118,7 +118,7 @@ func (s *StoreManager) GetMessage(mailbox, id string) (*Message, error) {
 		return nil, err
 	}
 	_ = r.Close()
-	header := makeMetadata(sm)
+	header := MakeMetadata(sm)
 	return &Message{MessageMetadata: *header, env: env}, nil
 }
 
@@ -153,8 +153,8 @@ func (s *StoreManager) MailboxForAddress(mailbox string) (string, error) {
 	return s.AddrPolicy.ExtractMailbox(mailbox)
 }
 
-// makeMetadata populates Metadata from a storage.Message.
-func makeMetadata(m storage.Message) *event.MessageMetadata {
+// MakeMetadata populates Metadata from a storage.Message.
+func MakeMetadata(m storage.Message) *event.MessageMetadata {
 	return &event.MessageMetadata{
 		Mailbox: m.Mailbox(),
 		ID:      m.ID(),
diff --git a/pkg/server/lifecycle.go b/pkg/server/lifecycle.go
index 757b397..a6b6faa 100644
--- a/pkg/server/lifecycle.go
+++ b/pkg/server/lifecycle.go
@@ -42,7 +42,7 @@ func FullAssembly(conf *config.Root) (*Services, error) {
 	}
 
 	// Configure storage.
-	store, err := storage.FromConfig(conf.Storage)
+	store, err := storage.FromConfig(conf.Storage, extHost)
 	if err != nil {
 		return nil, err
 	}
diff --git a/pkg/storage/file/fstore.go b/pkg/storage/file/fstore.go
index b08b021..d1e6231 100644
--- a/pkg/storage/file/fstore.go
+++ b/pkg/storage/file/fstore.go
@@ -10,6 +10,7 @@ import (
 	"time"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
 	"github.com/inbucket/inbucket/pkg/storage"
 	"github.com/inbucket/inbucket/pkg/stringutil"
 	"github.com/rs/zerolog/log"
@@ -45,10 +46,11 @@ type Store struct {
 	mailPath      string
 	messageCap    int
 	bufReaderPool sync.Pool
+	extHost       *extension.Host
 }
 
 // New creates a new DataStore object using the specified path
-func New(cfg config.Storage) (storage.Store, error) {
+func New(cfg config.Storage, extHost *extension.Host) (storage.Store, error) {
 	path := cfg.Params["path"]
 	if path == "" {
 		return nil, fmt.Errorf("'path' parameter not specified")
@@ -70,6 +72,7 @@ func New(cfg config.Storage) (storage.Store, error) {
 				return bufio.NewReader(nil)
 			},
 		},
+		extHost: extHost,
 	}, nil
 }
 
diff --git a/pkg/storage/file/fstore_test.go b/pkg/storage/file/fstore_test.go
index 71c6c48..bd325cd 100644
--- a/pkg/storage/file/fstore_test.go
+++ b/pkg/storage/file/fstore_test.go
@@ -14,6 +14,7 @@ import (
 	"time"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
 	"github.com/inbucket/inbucket/pkg/extension/event"
 	"github.com/inbucket/inbucket/pkg/message"
 	"github.com/inbucket/inbucket/pkg/storage"
@@ -23,18 +24,19 @@ import (
 
 // TestSuite runs storage package test suite on file store.
 func TestSuite(t *testing.T) {
-	test.StoreSuite(t, func(conf config.Storage) (storage.Store, func(), error) {
-		ds, _ := setupDataStore(conf)
-		destroy := func() {
-			teardownDataStore(ds)
-		}
-		return ds, destroy, nil
-	})
+	test.StoreSuite(t,
+		func(conf config.Storage, extHost *extension.Host) (storage.Store, func(), error) {
+			ds, _ := setupDataStore(conf, extHost)
+			destroy := func() {
+				teardownDataStore(ds)
+			}
+			return ds, destroy, nil
+		})
 }
 
 // Test directory structure created by filestore
 func TestFSDirStructure(t *testing.T) {
-	ds, logbuf := setupDataStore(config.Storage{})
+	ds, logbuf := setupDataStore(config.Storage{}, extension.NewHost())
 	defer teardownDataStore(ds)
 	root := ds.path
 
@@ -112,7 +114,7 @@ func TestFSDirStructure(t *testing.T) {
 
 // Test missing files
 func TestFSMissing(t *testing.T) {
-	ds, logbuf := setupDataStore(config.Storage{})
+	ds, logbuf := setupDataStore(config.Storage{}, extension.NewHost())
 	defer teardownDataStore(ds)
 
 	mbName := "fred"
@@ -147,7 +149,7 @@ func TestFSMissing(t *testing.T) {
 
 // Test Get the latest message
 func TestGetLatestMessage(t *testing.T) {
-	ds, logbuf := setupDataStore(config.Storage{})
+	ds, logbuf := setupDataStore(config.Storage{}, extension.NewHost())
 	defer teardownDataStore(ds)
 
 	// james hashes to 474ba67bdb289c6263b36dfd8a7bed6c85b04943
@@ -189,22 +191,25 @@ func TestGetLatestMessage(t *testing.T) {
 }
 
 // setupDataStore creates a new FileDataStore in a temporary directory
-func setupDataStore(cfg config.Storage) (*Store, *bytes.Buffer) {
+func setupDataStore(cfg config.Storage, extHost *extension.Host) (*Store, *bytes.Buffer) {
 	path, err := ioutil.TempDir("", "inbucket")
 	if err != nil {
 		panic(err)
 	}
+
 	// Capture log output.
 	buf := new(bytes.Buffer)
 	log.SetOutput(buf)
+
 	if cfg.Params == nil {
 		cfg.Params = make(map[string]string)
 	}
 	cfg.Params["path"] = path
-	s, err := New(cfg)
+	s, err := New(cfg, extHost)
 	if err != nil {
 		panic(err)
 	}
+
 	return s.(*Store), buf
 }
 
diff --git a/pkg/storage/file/mbox.go b/pkg/storage/file/mbox.go
index 42fe652..035f450 100644
--- a/pkg/storage/file/mbox.go
+++ b/pkg/storage/file/mbox.go
@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"sync"
 
+	"github.com/inbucket/inbucket/pkg/message"
 	"github.com/inbucket/inbucket/pkg/storage"
 	"github.com/rs/zerolog/log"
 )
@@ -72,6 +73,10 @@ func (mb *mbox) removeMessage(id string) error {
 			msg = m
 			// Slice around message we are deleting
 			mb.messages = append(mb.messages[:i], mb.messages[i+1:]...)
+
+			// Emit deleted event.
+			mb.store.extHost.Events.AfterMessageDeleted.Emit(message.MakeMetadata(msg))
+
 			break
 		}
 	}
diff --git a/pkg/storage/mem/store.go b/pkg/storage/mem/store.go
index 8b4d017..fd1ceae 100644
--- a/pkg/storage/mem/store.go
+++ b/pkg/storage/mem/store.go
@@ -8,6 +8,8 @@ import (
 	"sync"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
+	"github.com/inbucket/inbucket/pkg/message"
 	"github.com/inbucket/inbucket/pkg/storage"
 )
 
@@ -18,6 +20,7 @@ type Store struct {
 	cap      int           // Per-mailbox message cap.
 	incoming chan *msgDone // New messages for size enforcer.
 	remove   chan *msgDone // Remove deleted messages from size enforcer.
+	extHost  *extension.Host
 }
 
 type mbox struct {
@@ -31,10 +34,11 @@ type mbox struct {
 var _ storage.Store = &Store{}
 
 // New returns an emtpy memory store.
-func New(cfg config.Storage) (storage.Store, error) {
+func New(cfg config.Storage, extHost *extension.Host) (storage.Store, error) {
 	s := &Store{
-		boxes: make(map[string]*mbox),
-		cap:   cfg.MailboxMsgCap,
+		boxes:   make(map[string]*mbox),
+		cap:     cfg.MailboxMsgCap,
+		extHost: extHost,
 	}
 	if str, ok := cfg.Params["maxkb"]; ok {
 		maxKB, err := strconv.ParseInt(str, 10, 64)
@@ -163,6 +167,11 @@ func (s *Store) removeMessage(mailbox, id string) *Message {
 			delete(mb.messages, id)
 		}
 	})
+
+	if m != nil {
+		s.extHost.Events.AfterMessageDeleted.Emit(message.MakeMetadata(m))
+	}
+
 	return m
 }
 
diff --git a/pkg/storage/mem/store_test.go b/pkg/storage/mem/store_test.go
index 1fa27b7..efdf2bf 100644
--- a/pkg/storage/mem/store_test.go
+++ b/pkg/storage/mem/store_test.go
@@ -6,27 +6,32 @@ import (
 	"time"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
 	"github.com/inbucket/inbucket/pkg/storage"
 	"github.com/inbucket/inbucket/pkg/test"
 )
 
 // TestSuite runs storage package test suite on file store.
 func TestSuite(t *testing.T) {
-	test.StoreSuite(t, func(conf config.Storage) (storage.Store, func(), error) {
-		s, _ := New(conf)
-		destroy := func() {}
-		return s, destroy, nil
-	})
+	test.StoreSuite(t,
+		func(conf config.Storage, extHost *extension.Host) (storage.Store, func(), error) {
+			s, _ := New(conf, extHost)
+			destroy := func() {}
+			return s, destroy, nil
+		})
 }
 
 // TestMessageList verifies the operation of the global message list: mem.Store.messages.
 func TestMaxSize(t *testing.T) {
+	extHost := extension.NewHost()
 	maxSize := int64(2048)
-	s, _ := New(config.Storage{Params: map[string]string{"maxkb": "2"}})
+	s, _ := New(config.Storage{Params: map[string]string{"maxkb": "2"}}, extHost)
 	boxes := []string{"alpha", "beta", "whiskey", "tango", "foxtrot"}
+
+	// Ensure capacity so we do not block population.
 	n := 10
-	// total := 50
 	sizeChan := make(chan int64, len(boxes))
+
 	// Populate mailboxes concurrently.
 	for _, mailbox := range boxes {
 		go func(mailbox string) {
@@ -38,11 +43,13 @@ func TestMaxSize(t *testing.T) {
 			sizeChan <- size
 		}(mailbox)
 	}
+
 	// Wait for sizes.
 	sentBytesTotal := int64(0)
 	for range boxes {
 		sentBytesTotal += <-sizeChan
 	}
+
 	// Calculate actual size.
 	gotSize := int64(0)
 	s.VisitMailboxes(func(messages []storage.Message) bool {
@@ -51,6 +58,7 @@ func TestMaxSize(t *testing.T) {
 		}
 		return true
 	})
+
 	// Verify state. Messages are ~75 bytes each.
 	if gotSize < 2048-75 {
 		t.Errorf("Got total size %v, want greater than: %v", gotSize, 2048-75)
@@ -58,6 +66,7 @@ func TestMaxSize(t *testing.T) {
 	if gotSize > maxSize {
 		t.Errorf("Got total size %v, want less than: %v", gotSize, maxSize)
 	}
+
 	// Purge all messages concurrently, testing for deadlocks.
 	wg := &sync.WaitGroup{}
 	wg.Add(len(boxes))
diff --git a/pkg/storage/storage.go b/pkg/storage/storage.go
index d644429..a9c00ec 100644
--- a/pkg/storage/storage.go
+++ b/pkg/storage/storage.go
@@ -9,6 +9,7 @@ import (
 	"time"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
 )
 
 var (
@@ -19,7 +20,7 @@ var (
 	ErrNotWritable = errors.New("Message not writable")
 
 	// Constructors tracks registered storage constructors
-	Constructors = make(map[string]func(config.Storage) (Store, error))
+	Constructors = make(map[string]func(config.Storage, *extension.Host) (Store, error))
 )
 
 // Store is the interface Inbucket uses to interact with storage implementations.
@@ -48,9 +49,9 @@ type Message interface {
 }
 
 // FromConfig creates an instance of the Store based on the provided configuration.
-func FromConfig(c config.Storage) (store Store, err error) {
+func FromConfig(c config.Storage, extHost *extension.Host) (store Store, err error) {
 	if cf := Constructors[c.Type]; cf != nil {
-		return cf(c)
+		return cf(c, extHost)
 	}
 	return nil, fmt.Errorf("unknown storage type configured: %q", c.Type)
 }
diff --git a/pkg/test/integration_test.go b/pkg/test/integration_test.go
index 4f54ef7..60d29bc 100644
--- a/pkg/test/integration_test.go
+++ b/pkg/test/integration_test.go
@@ -217,6 +217,10 @@ func formatMessage(m *client.Message) []byte {
 func startServer() (func(), error) {
 	// TODO Move integration setup into lifecycle.
 	log.Logger = log.Output(zerolog.ConsoleWriter{Out: os.Stderr, NoColor: true})
+
+	extHost := extension.NewHost()
+
+	// Storage setup.
 	storage.Constructors["memory"] = mem.New
 	os.Clearenv()
 	conf, err := config.Process()
@@ -224,7 +228,7 @@ func startServer() (func(), error) {
 		return nil, err
 	}
 	svcCtx, svcCancel := context.WithCancel(context.Background())
-	store, err := storage.FromConfig(conf.Storage)
+	store, err := storage.FromConfig(conf.Storage, extHost)
 	if err != nil {
 		svcCancel()
 		return nil, err
@@ -232,7 +236,6 @@ func startServer() (func(), error) {
 
 	// TODO Test should not pass with unstarted msghub.
 	addrPolicy := &policy.Addressing{Config: conf}
-	extHost := extension.NewHost()
 	msgHub := msghub.New(conf.Web.MonitorHistory, extHost)
 	mmanager := &message.StoreManager{AddrPolicy: addrPolicy, Store: store, ExtHost: extHost}
 
diff --git a/pkg/test/storage_suite.go b/pkg/test/storage_suite.go
index 3befcdd..fa13260 100644
--- a/pkg/test/storage_suite.go
+++ b/pkg/test/storage_suite.go
@@ -10,19 +10,23 @@ import (
 	"time"
 
 	"github.com/inbucket/inbucket/pkg/config"
+	"github.com/inbucket/inbucket/pkg/extension"
 	"github.com/inbucket/inbucket/pkg/extension/event"
 	"github.com/inbucket/inbucket/pkg/message"
 	"github.com/inbucket/inbucket/pkg/storage"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 // StoreFactory returns a new store for the test suite.
-type StoreFactory func(config.Storage) (store storage.Store, destroy func(), err error)
+type StoreFactory func(
+	config.Storage, *extension.Host) (store storage.Store, destroy func(), err error)
 
 // StoreSuite runs a set of general tests on the provided Store.
 func StoreSuite(t *testing.T, factory StoreFactory) {
 	testCases := []struct {
 		name string
-		test func(*testing.T, storage.Store)
+		test func(*testing.T, storage.Store, *extension.Host)
 		conf config.Storage
 	}{
 		{"metadata", testMetadata, config.Storage{}},
@@ -40,18 +44,19 @@ func StoreSuite(t *testing.T, factory StoreFactory) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			store, destroy, err := factory(tc.conf)
+			extHost := extension.NewHost()
+			store, destroy, err := factory(tc.conf, extHost)
 			if err != nil {
 				t.Fatal(err)
 			}
-			tc.test(t, store)
+			tc.test(t, store, extHost)
 			destroy()
 		})
 	}
 }
 
 // testMetadata verifies message metadata is stored and retrieved correctly.
-func testMetadata(t *testing.T, store storage.Store) {
+func testMetadata(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "testmailbox"
 	from := &mail.Address{Name: "From Person", Address: "from@person.com"}
 	to := []*mail.Address{
@@ -118,7 +123,7 @@ func testMetadata(t *testing.T, store storage.Store) {
 }
 
 // testContent generates some binary content and makes sure it is correctly retrieved.
-func testContent(t *testing.T, store storage.Store) {
+func testContent(t *testing.T, store storage.Store, extHost *extension.Host) {
 	content := make([]byte, 5000)
 	for i := 0; i < len(content); i++ {
 		content[i] = byte(i % 256)
@@ -175,7 +180,7 @@ func testContent(t *testing.T, store storage.Store) {
 
 // testDeliveryOrder delivers several messages to the same mailbox, meanwhile querying its contents
 // with a new GetMessages call each cycle.
-func testDeliveryOrder(t *testing.T, store storage.Store) {
+func testDeliveryOrder(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "fred"
 	subjects := []string{"alpha", "bravo", "charlie", "delta", "echo"}
 	for i, subj := range subjects {
@@ -195,7 +200,7 @@ func testDeliveryOrder(t *testing.T, store storage.Store) {
 
 // testLatest delivers several messages to the same mailbox, and confirms the id `latest` returns
 // the last message sent.
-func testLatest(t *testing.T, store storage.Store) {
+func testLatest(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "fred"
 	subjects := []string{"alpha", "bravo", "charlie", "delta", "echo"}
 	for _, subj := range subjects {
@@ -217,14 +222,14 @@ func testLatest(t *testing.T, store storage.Store) {
 }
 
 // testNaming ensures the store does not enforce local part mailbox naming.
-func testNaming(t *testing.T, store storage.Store) {
+func testNaming(t *testing.T, store storage.Store, extHost *extension.Host) {
 	DeliverToStore(t, store, "fred@fish.net", "disk #27", time.Now())
 	GetAndCountMessages(t, store, "fred", 0)
 	GetAndCountMessages(t, store, "fred@fish.net", 1)
 }
 
 // testSize verifies message content size metadata values.
-func testSize(t *testing.T, store storage.Store) {
+func testSize(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "fred"
 	subjects := []string{"a", "br", "much longer than the others"}
 	sentIds := make([]string, len(subjects))
@@ -248,7 +253,7 @@ func testSize(t *testing.T, store storage.Store) {
 }
 
 // testSeen verifies a message can be marked as seen.
-func testSeen(t *testing.T, store storage.Store) {
+func testSeen(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "lisa"
 	id1, _ := DeliverToStore(t, store, mailbox, "whatever", time.Now())
 	id2, _ := DeliverToStore(t, store, mailbox, "hello?", time.Now())
@@ -284,22 +289,24 @@ func testSeen(t *testing.T, store storage.Store) {
 }
 
 // testDelete creates and deletes some messages.
-func testDelete(t *testing.T, store storage.Store) {
+func testDelete(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "fred"
 	subjects := []string{"alpha", "bravo", "charlie", "delta", "echo"}
 	for _, subj := range subjects {
 		DeliverToStore(t, store, mailbox, subj, time.Now())
 	}
 	msgs := GetAndCountMessages(t, store, mailbox, len(subjects))
+
+	// Subscribe to events.
+	eventListener := extHost.Events.AfterMessageDeleted.AsyncTestListener(2)
+
 	// Delete a couple messages.
-	err := store.RemoveMessage(mailbox, msgs[1].ID())
-	if err != nil {
-		t.Fatal(err)
-	}
-	err = store.RemoveMessage(mailbox, msgs[3].ID())
-	if err != nil {
-		t.Fatal(err)
+	deleteIDs := []string{msgs[1].ID(), msgs[3].ID()}
+	for _, id := range deleteIDs {
+		err := store.RemoveMessage(mailbox, id)
+		require.NoError(t, err)
 	}
+
 	// Confirm deletion.
 	subjects = []string{"alpha", "charlie", "echo"}
 	msgs = GetAndCountMessages(t, store, mailbox, len(subjects))
@@ -309,6 +316,17 @@ func testDelete(t *testing.T, store storage.Store) {
 			t.Errorf("Got subject %q, want %q", got, want)
 		}
 	}
+
+	// Capture events and check correct IDs were emitted.
+	ev1, err := eventListener()
+	require.NoError(t, err)
+	ev2, err := eventListener()
+	require.NoError(t, err)
+	eventIDs := []string{ev1.ID, ev2.ID}
+	for _, id := range deleteIDs {
+		assert.Contains(t, eventIDs, id)
+	}
+
 	// Try appending one more.
 	DeliverToStore(t, store, mailbox, "foxtrot", time.Now())
 	subjects = []string{"alpha", "charlie", "echo", "foxtrot"}
@@ -322,7 +340,7 @@ func testDelete(t *testing.T, store storage.Store) {
 }
 
 // testPurge makes sure mailboxes can be purged.
-func testPurge(t *testing.T, store storage.Store) {
+func testPurge(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "fred"
 	subjects := []string{"alpha", "bravo", "charlie", "delta", "echo"}
 	for _, subj := range subjects {
@@ -338,7 +356,7 @@ func testPurge(t *testing.T, store storage.Store) {
 }
 
 // testMsgCap verifies the message cap is enforced.
-func testMsgCap(t *testing.T, store storage.Store) {
+func testMsgCap(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mbCap := 10
 	mailbox := "captain"
 	for i := 0; i < 20; i++ {
@@ -365,7 +383,7 @@ func testMsgCap(t *testing.T, store storage.Store) {
 }
 
 // testNoMsgCap verfies a cap of 0 is not enforced.
-func testNoMsgCap(t *testing.T, store storage.Store) {
+func testNoMsgCap(t *testing.T, store storage.Store, extHost *extension.Host) {
 	mailbox := "captain"
 	for i := 0; i < 20; i++ {
 		subj := fmt.Sprintf("subject %v", i)
@@ -376,7 +394,7 @@ func testNoMsgCap(t *testing.T, store storage.Store) {
 
 // testVisitMailboxes creates some mailboxes and confirms the VisitMailboxes method visits all of
 // them.
-func testVisitMailboxes(t *testing.T, ds storage.Store) {
+func testVisitMailboxes(t *testing.T, ds storage.Store, extHost *extension.Host) {
 	boxes := []string{"abby", "bill", "christa", "donald", "evelyn"}
 	for _, name := range boxes {
 		DeliverToStore(t, ds, name, "Old Message", time.Now().Add(-24*time.Hour))
diff --git a/shell.nix b/shell.nix
index f1c5931..263b912 100644
--- a/shell.nix
+++ b/shell.nix
@@ -15,6 +15,7 @@ pkgs.mkShell {
   buildInputs = with pkgs; [
     act
     dpkg
+    delve
     elmPackages.elm
     elmPackages.elm-analyse
     elmPackages.elm-format