diff --git a/etc/swaks-tests/mime-errors.raw b/etc/swaks-tests/mime-errors.raw
new file mode 100644
index 0000000..c00fd09
--- /dev/null
+++ b/etc/swaks-tests/mime-errors.raw
@@ -0,0 +1,31 @@
+Date: %DATE%
+To: %TO_ADDRESS%
+From: %FROM_ADDRESS%
+Subject: MIME Errors
+Message-Id: <07B7061D-2676-487E-942E-C341CE4D13DC@makita.skynet>
+Content-Type: multipart/alternative; boundary="Enmime-Test-100"
+
+--Enmime-Test-100
+Content-Transfer-Encoding: 8bit
+Content-Type: text/plain; charset=us-ascii
+
+Using Unicode/UTF-8, you can write in emails and source code things such as
+
+Mathematics and sciences:
+
+  ∮ E⋅da = Q,  n → ∞, ∑ f(i) = ∏ g(i),      ⎧⎡⎛┌─────┐⎞⎤⎫
+                                            ⎪⎢⎜│a²+b³ ⎟⎥⎪
+  ∀x∈ℝ: ⌈x⌉ = −⌊−x⌋, α ∧ ¬β = ¬(¬α ∨ β),    ⎪⎢⎜│───── ⎟⎥⎪
+                                            ⎪⎢⎜⎷ c₈   ⎟⎥⎪
+  ℕ ⊆ ℕ₀ ⊂ ℤ ⊂ ℚ ⊂ ℝ ⊂ ℂ,                   ⎨⎢⎜       ⎟⎥⎬
+                                            ⎪⎢⎜ ∞     ⎟⎥⎪
+  ⊥ < a ≠ b ≡ c ≤ d ≪ ⊤ ⇒ (⟦A⟧ ⇔ ⟪B⟫),      ⎪⎢⎜ ⎲     ⎟⎥⎪
+                                            ⎪⎢⎜ ⎳aⁱ-bⁱ⎟⎥⎪
+  2H₂ + O₂ ⇌ 2H₂O, R = 4.7 kΩ, ⌀ 200 mm     ⎩⎣⎝i=1    ⎠⎦⎭
+
+Linguistics and dictionaries:
+
+  ði ıntəˈnæʃənəl fəˈnɛtık əsoʊsiˈeıʃn
+  Y [ˈʏpsilɔn], Yen [jɛn], Yoga [ˈjoːgɑ]
+
+--Enmime-Test-100
diff --git a/etc/swaks-tests/run-tests.sh b/etc/swaks-tests/run-tests.sh
index c9a93b8..548454b 100755
--- a/etc/swaks-tests/run-tests.sh
+++ b/etc/swaks-tests/run-tests.sh
@@ -56,3 +56,6 @@ swaks $* --data outlook.raw
 # Non-mime responsive HTML test
 swaks $* --data nonmime-html-responsive.raw
 swaks $* --data nonmime-html-inlined.raw
+
+# Incorrect charset, malformed final boundary
+swaks $* --data mime-errors.raw
diff --git a/pkg/webui/mailbox_controller.go b/pkg/webui/mailbox_controller.go
index c9423e1..d1fc80c 100644
--- a/pkg/webui/mailbox_controller.go
+++ b/pkg/webui/mailbox_controller.go
@@ -6,7 +6,6 @@ import (
 	"io"
 	"net/http"
 	"strconv"
-	"time"
 
 	"github.com/jhillyerd/inbucket/pkg/server/web"
 	"github.com/jhillyerd/inbucket/pkg/storage"
@@ -15,30 +14,6 @@ import (
 	"github.com/rs/zerolog/log"
 )
 
-// JSONMessage formats message data for the UI.
-type JSONMessage struct {
-	Mailbox     string              `json:"mailbox"`
-	ID          string              `json:"id"`
-	From        string              `json:"from"`
-	To          []string            `json:"to"`
-	Subject     string              `json:"subject"`
-	Date        time.Time           `json:"date"`
-	PosixMillis int64               `json:"posix-millis"`
-	Size        int64               `json:"size"`
-	Seen        bool                `json:"seen"`
-	Header      map[string][]string `json:"header"`
-	Text        string              `json:"text"`
-	HTML        string              `json:"html"`
-	Attachments []*JSONAttachment   `json:"attachments"`
-}
-
-// JSONAttachment formats attachment data for the UI.
-type JSONAttachment struct {
-	ID          string `json:"id"`
-	FileName    string `json:"filename"`
-	ContentType string `json:"content-type"`
-}
-
 // MailboxMessage outputs a particular message as JSON for the UI.
 func MailboxMessage(w http.ResponseWriter, req *http.Request, ctx *web.Context) (err error) {
 	id := ctx.Vars["id"]
@@ -54,15 +29,25 @@ func MailboxMessage(w http.ResponseWriter, req *http.Request, ctx *web.Context)
 		http.NotFound(w, req)
 		return nil
 	}
-	attachParts := msg.Attachments()
-	attachments := make([]*JSONAttachment, len(attachParts))
-	for i, part := range attachParts {
-		attachments[i] = &JSONAttachment{
+
+	attachments := make([]*jsonAttachment, 0)
+	for i, part := range msg.Attachments() {
+		attachments = append(attachments, &jsonAttachment{
 			ID:          strconv.Itoa(i),
 			FileName:    part.FileName,
 			ContentType: part.ContentType,
-		}
+		})
 	}
+
+	mimeErrors := make([]*jsonMIMEError, 0)
+	for _, e := range msg.MIMEErrors() {
+		mimeErrors = append(mimeErrors, &jsonMIMEError{
+			Name:   e.Name,
+			Detail: e.Detail,
+			Severe: e.Severe,
+		})
+	}
+
 	// Sanitize HTML body.
 	htmlBody := ""
 	if msg.HTML() != "" {
@@ -74,8 +59,9 @@ func MailboxMessage(w http.ResponseWriter, req *http.Request, ctx *web.Context)
 				Msg("HTML sanitizer failed")
 		}
 	}
+
 	return web.RenderJSON(w,
-		&JSONMessage{
+		&jsonMessage{
 			Mailbox:     name,
 			ID:          msg.ID,
 			From:        msg.From.String(),
@@ -89,6 +75,7 @@ func MailboxMessage(w http.ResponseWriter, req *http.Request, ctx *web.Context)
 			Text:        web.TextToHTML(msg.Text()),
 			HTML:        htmlBody,
 			Attachments: attachments,
+			Errors:      mimeErrors,
 		})
 }
 
diff --git a/pkg/webui/mailbox_json.go b/pkg/webui/mailbox_json.go
new file mode 100644
index 0000000..70252c5
--- /dev/null
+++ b/pkg/webui/mailbox_json.go
@@ -0,0 +1,34 @@
+package webui
+
+import "time"
+
+// jsonMessage formats message data for the UI.
+type jsonMessage struct {
+	Mailbox     string              `json:"mailbox"`
+	ID          string              `json:"id"`
+	From        string              `json:"from"`
+	To          []string            `json:"to"`
+	Subject     string              `json:"subject"`
+	Date        time.Time           `json:"date"`
+	PosixMillis int64               `json:"posix-millis"`
+	Size        int64               `json:"size"`
+	Seen        bool                `json:"seen"`
+	Header      map[string][]string `json:"header"`
+	Text        string              `json:"text"`
+	HTML        string              `json:"html"`
+	Attachments []*jsonAttachment   `json:"attachments"`
+	Errors      []*jsonMIMEError    `json:"errors"`
+}
+
+// jsonAttachment formats attachment data for the UI.
+type jsonAttachment struct {
+	ID          string `json:"id"`
+	FileName    string `json:"filename"`
+	ContentType string `json:"content-type"`
+}
+
+type jsonMIMEError struct {
+	Name   string `json:"name"`
+	Detail string `json:"detail"`
+	Severe bool   `json:"severe"`
+}