Sending as alias via relay when not running on standard port

[Kasper Peeters]

First a description of what does work: I have a satellite system which relays all outgoing mail to my mail-in-a-box server. That is, on the satellite I have

relayhost = [miab.domain.com]:587

and the appropriate username/password in /etc/postfix/sasl/passwd,

[miab.domain.com]:587 [e-mail]:securepassword

If I setup an alias [e-mail] on my miab server and give [e-mail] permission to send as that alias, this works fine, and the satellite can send email claiming to be from that alias. All good.

However, if I now open a second submission port, by adding to /etc/postfix/master.cf the line

 5870      inet  n       -       y       -       -       smtpd

and change 587 -> 5870 in the configuration on the satellite system, then I can still send email from the satellite system as [e-mail], but only to domains served by the miab server. It refuses to send emails to the rest of the world, with a Relay access denied message.

Any ideas? Should I do something different to enable posting on a non-standard port? (I need this because the satellite system has outgoing traffic to port 587 blocked).

Apologies if this is not a mail-in-a-box issue.