Allow SMTP to Office 365 (smtp.office365.com)
Your blocking of standard SMTP ports seems reasonable until you realize that outgoing e-mail can still be sent through G Suite mail servers and not Office 365 mail servers. My domain e-mail that I want to send mail through (low volume) is hosted at Office 365. I'm not going to move it to G Suite just for one little use. I don't see what, if anything, this has to do with IP reputation since smtp.office365.com requires login information. I was excited to use Google Compute Engine until I discovered this annoying fact. Now I'm going to have to use AWS or Softlayer.
Mats Planberg commented
I agree with you Steven. What a petty way by Google to shut out Office 365 users? I had everything set up and was doing thru some last tests before going into production and didn't first understand why emails couldn't be sent. Now I will have to look at an Azure server instead. Too bad, I liked GCP, but this is not acceptable.