Originally Posted by TheDoc

Many people think it's gmail blocking them, but they are able to email project-emails from the mail server to gmail, without issues.

This is because the format/layout text-wording on the emails being filtered isn't matching natural writing styles. It sounds like a promo/offer, rather than info/fun/text email that most people get.

I would check to see if it's that, if not change the IP/mail server and then change your email format to stop it from happening again. If it looks like spam, smells like spam, even if it's opt-in, it will get marked as spam.