I connect to my Zimbra server from my mobile phone when I roam (off the LAN, on the cellular network, from some 'random' dynamically assigned IP), for both sending (SMTP) and receiving (IMAP). The connections are TLS encrypted and Authenticated. Pretty typical.

Each time I send a message from my roaming connected phone, I get in the logs

May  3 08:30:26 cheetah amavis[27180]: (27180-02) Open relay? Nonlocal recips but not originating: ###@###.com
I tested my server, and it's not an OpenRelay, but amavis thinks it is anyway.

I only get this message from a roaming connection.

I need to configure something I guess. How can I fix this?