I once spent a week on this problem spinning up a QA server, our application had an email function for sending receipts. Our company used GApps and the QA team was complaining of not getting the generated email receipts. I poured through every configuration and every key file, mail still failed from the server citing spam/verification problems from the GApp side.

A week of troubleshooting and we discovered GApps didn't like emails coming from Linode IP addresses.

A sprint and a half later we had rewritten our email functions to use sendgrid. An extra expense, extra dev time burned. Becaus we had done everything right but upstream it still wasn't good enough.