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Have anyone got SmarterMail and Gmail for business worked together and achieved hosting subset of users on each? Is there any known obstacles that might cause any delivery issues?
I have tried my personal parked domain to test that and having few configuration issues and sending to my legacy SmarterMail server from my active Gmail business account returns 550 Authentication is required for relay.
Kindly let me know if that worked so I keep searching for solution or not.