we run a zimbra server with domain : abc.com . it works great but now my boss wanna have a gmail account with domain "abc.com"( a service of google email for business). the problem is we wanna keep both zimbra server and my boss's gmail account.only my boss use his gmail account but all user from my company still use zimbra. can you guys suggest me any solution ? thanks in advance !

p.s: i read about "relay over external server" but it cant solve my problem when i need both ingoing and going via gmail for a few of account.