Hi Jorge,
I have exactly the same problem. I no mere use a relay server and for some domais, it appears to try to send over relay server. I can't understood, why just some?
I ran the zmprov gs 'zimbra.server' | grep zimbraMtaSmtpSaslPasswordMaps and it shows me a result, then I ran zmprov ms 'zimbra.server' zimbraMtaSmtpSaslPasswordMaps "" as you said, restarted Zimbra, but it also try to send to particular domains relaying.
Extracted from zimbra.log
warning: hash:/opt/zimbra/conf/relay_password lookup error for "mail.quimesp.com"
How can I fix it?
Best Regards
Ricardo