zmprov modifyAccount firstname.lastname@example.org +zimbraMailCatchAllAddress @otherdomain.com
Do they have any admin permissions at all? (zimbra admin console gui) You could use soap:
If you want to set up an account „email@example.com“ to catch any mail not delivered to existing users in the domain „domain.com“, you can configure the account as a domain catchall.
If the users „firstname.lastname@example.org“, „email@example.com“, and „firstname.lastname@example.org“ don’t exist, and mail arrives for them, it will be delivered to the catchall account „email@example.com“. This will increase the amount of spam delivered, and can lead to being blacklisted. To remove the catchall from an email account, unset the catchall address: