in smtpserver, accept delivery to postmaster@<hostname>, and also postmaster@ addresses for domains that don't have a postmaster address configured.

.gitignore

@@ -17,6 +17,7 @@
 /testdata/smtp/datajunk/
 /testdata/smtp/sendlimit/data/
 /testdata/smtp/catchall/data/
+/testdata/smtp/postmaster/
 /testdata/store/data/
 /testdata/train/
 /cover.out

config/config.go

@@ -52,7 +52,7 @@ type Static struct {
 	Postmaster        struct {
 		Account string
 		Mailbox string `sconf-doc:"E.g. Postmaster or Inbox."`
-	} `sconf-doc:"Destination for emails delivered to postmaster address."`
+	} `sconf-doc:"Destination for emails delivered to postmaster addresses: a plain 'postmaster' without domain, 'postmaster@<hostname>' (also for each listener with SMTP enabled), and as fallback for each domain without explicitly configured postmaster destination."`
 	DefaultMailboxes []string `sconf:"optional" sconf-doc:"Mailboxes to create when adding an account. Inbox is always created. If no mailboxes are specified, the following are automatically created: Sent, Archive, Trash, Drafts and Junk."`
 
 	// All IPs that were explicitly listen on for external SMTP. Only set when there

config/doc.go

@@ -321,7 +321,10 @@ describe-static" and "mox config describe-domains":
 				# Port for HTTPS webserver. (optional)
 				Port: 0
 
-	# Destination for emails delivered to postmaster address.
+	# Destination for emails delivered to postmaster addresses: a plain 'postmaster'
+	# without domain, 'postmaster@<hostname>' (also for each listener with SMTP
+	# enabled), and as fallback for each domain without explicitly configured
+	# postmaster destination.
 	Postmaster:
 		Account:
 

mox-/admin.go

@@ -270,7 +270,7 @@ func MakeDomainConfig(ctx context.Context, domain, hostname dns.Domain, accountN
 // DomainAdd adds the domain to the domains config, rewriting domains.conf and
 // marking it loaded.
 //
-// accountName is used for DMARC/TLS report.
+// accountName is used for DMARC/TLS report and potentially for the postmaster address.
 // If the account does not exist, it is created with localpart. Localpart must be
 // set only if the account does not yet exist.
 func DomainAdd(ctx context.Context, domain dns.Domain, accountName string, localpart smtp.Localpart) (rerr error) {
@@ -325,6 +325,16 @@ func DomainAdd(ctx context.Context, domain dns.Domain, accountName string, local
 		return fmt.Errorf("account name is empty")
 	} else if !ok {
 		nc.Accounts[accountName] = MakeAccountConfig(smtp.Address{Localpart: localpart, Domain: domain})
+	} else if accountName != Conf.Static.Postmaster.Account {
+		nacc := nc.Accounts[accountName]
+		nd := map[string]config.Destination{}
+		for k, v := range nacc.Destinations {
+			nd[k] = v
+		}
+		pmaddr := smtp.Address{Localpart: "postmaster", Domain: domain}
+		nd[pmaddr.String()] = config.Destination{}
+		nacc.Destinations = nd
+		nc.Accounts[accountName] = nacc
 	}
 
 	nc.Domains[domain.Name()] = confDomain

mox-/lookup.go

@@ -22,8 +22,21 @@ func FindAccount(localpart smtp.Localpart, domain dns.Domain, allowPostmaster bo
 	if strings.EqualFold(string(localpart), "postmaster") {
 		localpart = "postmaster"
 	}
+
+	postmasterDomain := func() bool {
 		var zerodomain dns.Domain
-	if localpart == "postmaster" && domain == zerodomain {
+		if domain == zerodomain || domain == Conf.Static.HostnameDomain {
+			return true
+		}
+		for _, l := range Conf.Static.Listeners {
+			if l.SMTP.Enabled && domain == l.HostnameDomain {
+				return true
+			}
+		}
+		return false
+	}
+
+	if localpart == "postmaster" && postmasterDomain() {
 		if !allowPostmaster {
 			return "", "", config.Destination{}, ErrAccountNotFound
 		}
@@ -44,6 +57,9 @@ func FindAccount(localpart smtp.Localpart, domain dns.Domain, allowPostmaster bo
 	accAddr, ok := Conf.AccountDestination(canonical)
 	if !ok {
 		if accAddr, ok = Conf.AccountDestination("@" + domain.Name()); !ok {
+			if localpart == "postmaster" && allowPostmaster {
+				return Conf.Static.Postmaster.Account, "postmaster", config.Destination{Mailbox: Conf.Static.Postmaster.Mailbox}, nil
+			}
 			return "", "", config.Destination{}, ErrAccountNotFound
 		}
 		canonical = "@" + domain.Name()

smtpserver/server_test.go

@@ -1097,3 +1097,37 @@ test email
 	testSubmit("mjl@mox.example", "mjl@mox.example")
 	testSubmit("mjl@mox.example", "mjl@mox2.example") // DKIM signature will be for mox2.example.
 }
+
+// Test to postmaster addresses.
+func TestPostmaster(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/postmaster/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 := "mjl@other.example"
+			if err == nil {
+				err = client.Deliver(context.Background(), 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("postmaster", nil)                  // Plain postmaster address without domain.
+	testDeliver("postmaster@host.mox.example", nil) // Postmaster address with configured mail server hostname.
+	testDeliver("postmaster@mox.example", nil)      // Postmaster address without explicitly configured destination.
+	testDeliver("postmaster@unknown.example", &smtpclient.Error{Code: smtp.C550MailboxUnavail, Secode: smtp.SeAddr1UnknownDestMailbox1})
+}

testdata/smtp/postmaster/domains.conf

@@ -0,0 +1,7 @@
+Domains:
+	mox.example: nil
+Accounts:
+	mjl:
+		Domain: mox.example
+		Destinations:
+			mjl@mox.example: nil

testdata/smtp/postmaster/mox.conf

@@ -0,0 +1,9 @@
+DataDir: data
+User: 1000
+LogLevel: trace
+Hostname: host.mox.example
+Postmaster:
+	Account: mjl
+	Mailbox: postmaster
+Listeners:
+	local: nil