diff --git a/core/admin/mailu/internal/views/postfix.py b/core/admin/mailu/internal/views/postfix.py
index 21a4aa91..0843b147 100644
--- a/core/admin/mailu/internal/views/postfix.py
+++ b/core/admin/mailu/internal/views/postfix.py
@@ -36,7 +36,11 @@ def postfix_transport(email):
         return flask.abort(404)
     localpart, domain_name = models.Email.resolve_domain(email)
     relay = models.Relay.query.get(domain_name) or flask.abort(404)
-    return flask.jsonify("smtp:[{}]".format(relay.smtp))
+    ret = "smtp:[{0}]".format(relay.smtp)
+    if ":" in relay.smtp:
+        split = relay.smtp.split(':')
+        ret = "smtp:[{0}]:{1}".format(split[0], split[1])
+    return flask.jsonify(ret)
 
 
 @internal.route("/postfix/sender/login/<path:sender>")
diff --git a/towncrier/newsfragments/1357.feature b/towncrier/newsfragments/1357.feature
new file mode 100644
index 00000000..56fa443f
--- /dev/null
+++ b/towncrier/newsfragments/1357.feature
@@ -0,0 +1 @@
+Relay a domain to a nonstandard SMTP port by adding ":<port_num>" to the remote hostname or IP address.
\ No newline at end of file