because, one day, you will have too many customers, and generate more emails than the great Google deems appropriate for your company.
550 5.4.5 Daily sending quota exceeded.
I would have to say that the smtp gmail service isn't reliable anyway with it's two second response times,
421 4.7.0 Temporary System Problem. Try again later (WS).and
550 5.7.0 Mail Sending denied.all up in my face.
Switching to a local postfix server tonight. 5 emails/second, no errors. Much nicer.