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.
We are investigating this.
You can actually bypass that restriction using Microsoft Flow. You can set up a Flow that will send an email from a post request over https (port 443) so the outgoing port from the Google machine will not be SMTP. It is a really small change if you build you code correctly and you created your own sending method. Just send the call to the smtp sending request to a POST request to the url you got from your brand new Flow.
Just remember to add auth to the flow. I use a key and compare it to a 50 letters value to make sure. It cannot be sniffed as the request is encrypted over SSL.
Aman Brar commented
Isn't this hugely anti-competition? Mailgun, SendGrid etc are all allowed but not Office 365? We're planning to launch a CRM integrated with our office365 mailboxes and this is a huge pain.
I indeed just figured out that my Google Cloud VPS does not support my Office365 mailserver. What a b...****. I am not willing to move to a paid G Suite service where I already pay for my Office365 environment
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.