I have a contact form on my website that uses SMTP to send a confirmation message to a few different email addresses. Every single email address hosted elsewhere receives these emails, verifies them, and displays them in my inbox.
However the email address hosted at office365 doesn't show this email in any folder, not inbox, not spam, not deleted, not grouped, nothing. The address is not on my blocked list, in fact i've even added it to my "Safe Sender" list as well as my contacts and checked "Trust emails from my contacts". Nothing works.
Is there a firewall setting somewhere that is blocking this before anything else happens? I can't find anything for the life of me, but it's the only thing remaining I can fathom to try/ask about.
In case anyone googles this in the future - here's the issue and resolution.
Office 365 (no one else) blocks email services that send using "local mail exchanges" when their email MX records point externally. In our case, we were sending fully valid SMTP mail through a server who's IP did not match the domain's IP. Inside of cPanel we had to change the Email Routing to "Remote mail exchanger". Only then did the emails show up inside of Outlook at all.