Example:

Zimbra domain is set up as:
zimbra.whatever.com

Users are:
username@zimbra.whatever.com

However, when sending mail out, we want it to show up as:
username@whatever.com

Here’s the command to make this work:

zmprov md zimbra.whatever.com zimbraMailCatchAllAddress @zimbra.whatever.com zimbraMailCatchAllCanonicalAddress @whatever.com