Today in unintended-consequences land: If you send an email from Gmail to a SMTP server hosted on Google Cloud Platform ... it will send over an internal, private IP, not from the public Gmail IPs. So the SPF check will fail on those emails since SPF is a sender IP check. 1/2
sadly this isn’t new. this has been the case for the last year (at least that was when I first encountered this problem). In some cases the e-mails won’t even appear in the mailbox. It’s just sad that such huge company can’t fix such small problems