fix delivery from/to smtp addresses with double quotes

found while adding tests for smtp and imap for address with empty double (double
quoted) localparts.

imapserver/server_test.go

@@ -387,8 +387,12 @@ func TestLogin(t *testing.T) {
 	tc.close()
 
 	tc = start(t)
-	defer tc.close()
 	tc.transactf("ok", `login "mjl@mox.example" "testtest"`)
+	tc.close()
+
+	tc = start(t)
+	tc.transactf("ok", `login "\"\"@mox.example" "testtest"`)
+	defer tc.close()
 
 	tc.transactf("bad", "logout badarg")
 	tc.transactf("ok", "logout")

smtpserver/parse.go

@@ -336,6 +336,7 @@ func (p *parser) xlocalpart() smtp.Localpart {
 
 // ../rfc/5321:2324
 func (p *parser) xquotedString(islocalpart bool) string {
+	p.xtake(`"`)
 	var s string
 	var esc bool
 	for {

smtpserver/server_test.go

@@ -1133,3 +1133,34 @@ func TestPostmaster(t *testing.T) {
 	testDeliver("postmaster@mox.example", nil)      // Postmaster address without explicitly configured destination.
 	testDeliver("postmaster@unknown.example", &smtpclient.Error{Code: smtp.C550MailboxUnavail, Secode: smtp.SeAddr1UnknownDestMailbox1})
 }
+
+// Test to address with empty localpart.
+func TestEmptylocalpart(t *testing.T) {
+	resolver := dns.MockResolver{
+		A: map[string][]string{
+			"other.example.": {"127.0.0.10"}, // For mx check.
+		},
+		PTR: map[string][]string{
+			"127.0.0.10": {"other.example."},
+		},
+	}
+	ts := newTestServer(t, "../testdata/smtp/mox.conf", resolver)
+	defer ts.close()
+
+	testDeliver := func(rcptTo string, expErr *smtpclient.Error) {
+		t.Helper()
+		ts.run(func(err error, client *smtpclient.Client) {
+			t.Helper()
+			mailFrom := `""@other.example`
+			if err == nil {
+				err = client.Deliver(ctxbg, mailFrom, rcptTo, int64(len(deliverMessage)), strings.NewReader(deliverMessage), false, false)
+			}
+			var cerr smtpclient.Error
+			if expErr == nil && err != nil || expErr != nil && (err == nil || !errors.As(err, &cerr) || cerr.Code != expErr.Code || cerr.Secode != expErr.Secode) {
+				t.Fatalf("got err %#v, expected %#v", err, expErr)
+			}
+		})
+	}
+
+	testDeliver(`""@mox.example`, nil)
+}

testdata/imap/domains.conf

@@ -6,6 +6,7 @@ Accounts:
 		Domain: mox.example
 		Destinations:
 			mjl@mox.example: nil
+			""@mox.example: nil
 		JunkFilter:
 			Threshold: 0.95
 			Params:

testdata/smtp/domains.conf

@@ -7,6 +7,7 @@ Accounts:
 		Destinations:
 			mjl@mox.example: nil
 			mjl@mox2.example: nil
+			""@mox.example: nil
 		JunkFilter:
 			Threshold: 0.9
 			Params: