Code:
su - zimbra
zmprov gacf zimbraMtaMyDestination
you will probably find that this only includes localhost, so to add your mail server you can do
Code:
su - zimbra
zmprov mcf zimbraMtaMyDestination 'localhost, additional.domain.com'
/opt/zimbra/libexec/zmmtaconfig mta
postconf mydestination
should now include your additional domain.