I have 2 clients setup on G Suite. According to Google's support, their online tools, and everything I know about email delivery, the DNS configuration is perfect: SFP, DKIM, DMARC.
They can send email perfectly, and their email always arrives.
The problem is that they don't receive email for just a few domains. One is icloud, the other is a major hotel chain, and the other is a university.
I contacted Apple support and after literally months of trying to reach the right person, they were able to modify "something" on their end so emails will go through to one of the domains. (But they won't tell me what was wrong.)
Here's where it gets strange... the client that can now receive email from icloud can ONLY receive on one domain alias. The other domain aliases still do not get email from icloud or these other senders. So whatever Apple changed on their end only impacted this one domain.
Even stranger. If icloud sends an email to my.account@gmail.com, it goes through. If they send to my.account@client.com it does not go through. If they email BOTH gmail and CC the client, then neither email goes through? What in the WORLD could be the cause of something like this?
I can get the message ID from icloud, and GSuite support says those IDs never reach the server. Since I can't reach IT departments for any of these organizations, I can get any error codes or replies. I just have angry clients who are not receiving emails from some important contacts.