chasquid: Fail at RCPT TO time if a user does not exist

It's more convenient and in line with standard practice to fail RCPT TO if the
user does not exist.

This involves making the server and client aware of aliases, but it doesn't
end up being very convoluted, and simplifies other code.

internal/aliases/aliases_test.go

@@ -25,6 +25,22 @@ func (cases Cases) check(t *testing.T, r *Resolver) {
 	}
 }
 
+func mustExist(t *testing.T, r *Resolver, addrs ...string) {
+	for _, addr := range addrs {
+		if _, ok := r.Exists(addr); !ok {
+			t.Errorf("address %q does not exist, it should", addr)
+		}
+	}
+}
+
+func mustNotExist(t *testing.T, r *Resolver, addrs ...string) {
+	for _, addr := range addrs {
+		if _, ok := r.Exists(addr); ok {
+			t.Errorf("address %q exists, it should not", addr)
+		}
+	}
+}
+
 func TestBasic(t *testing.T) {
 	resolver := NewResolver()
 	resolver.aliases = map[string][]Recipient{
@@ -39,6 +55,9 @@ func TestBasic(t *testing.T) {
 		{"x@y", []Recipient{{"x@y", EMAIL}}},
 	}
 	cases.check(t, resolver)
+
+	mustExist(t, resolver, "a@b", "e@f", "cmd")
+	mustNotExist(t, resolver, "x@y")
 }
 
 func TestAddrRewrite(t *testing.T) {
@@ -81,6 +100,48 @@ func TestAddrRewrite(t *testing.T) {
 	cases.check(t, resolver)
 }
 
+func TestExistsRewrite(t *testing.T) {
+	resolver := NewResolver()
+	resolver.AddDomain("def")
+	resolver.AddDomain("p-q.com")
+	resolver.aliases = map[string][]Recipient{
+		"abc@def":  {{"x@y", EMAIL}},
+		"ñoño@def": {{"x@y", EMAIL}},
+		"recu@def": {{"ab+cd@p-q.com", EMAIL}},
+	}
+	resolver.DropChars = ".~"
+	resolver.SuffixSep = "-+"
+
+	mustExist(t, resolver, "abc@def", "a.bc+blah@def", "ño.ño@def")
+	mustNotExist(t, resolver, "abc@d.ef", "nothere@def")
+
+	cases := []struct {
+		addr         string
+		expectAddr   string
+		expectExists bool
+	}{
+		{"abc@def", "abc@def", true},
+		{"abc+blah@def", "abc@def", true},
+		{"a.b~c@def", "abc@def", true},
+		{"a.bc+blah@def", "abc@def", true},
+
+		{"a.bc@unknown", "a.bc@unknown", false},
+		{"x.yz@def", "xyz@def", false},
+		{"x.yz@d.ef", "x.yz@d.ef", false},
+	}
+	for _, c := range cases {
+		addr, exists := resolver.Exists(c.addr)
+		if addr != c.expectAddr {
+			t.Errorf("%q: expected addr %q, got %q",
+				c.addr, c.expectAddr, addr)
+		}
+		if exists != c.expectExists {
+			t.Errorf("%q: expected exists %v, got %v",
+				c.addr, c.expectExists, exists)
+		}
+	}
+}
+
 func TestTooMuchRecursion(t *testing.T) {
 	resolver := Resolver{}
 	resolver.aliases = map[string][]Recipient{