Outgoing Mail Problems
Are you sure you don't want SMTP Authentication?
Modifying the relay networks
If you want to allow additional hosts or networks to send email through your MTA, you add them to the allowed relay list. Be careful, misconfiguration of this can allow malicious outsiders to send spam through your system.
Getting the existing value
Run these commands on your MTA server:
su - zimbra zmprov gs `zmhostname` | grep zimbraMtaMyNetworks
If nothing is returned, you're using the postfix default - fetch that with:
su - zimbra postconf -d mynetworks
Setting the new value
To add hosts, you'll need to add to the existing value you just fetched:
su - zimbra zmprov ms `zmhostname` zimbraMtaMyNetworks "127.0.0.0/8 10.1.0.0/16 a.b.c.0/24 w.x.y.z/32"
The quotes above are important.
Restart the services when done.