There are days I think we have trained people too well to believe every delivery problem is a misplaced spam block. We also have people trained to expect near 100% immediate delivery from send to inbox.
The problem is, email isn’t 100% reliable. It’s close. Very close. But sometimes mail just fails. It’s not because the ISPs hate you. It’s sometimes not even because the mail looks like spam.
Sometimes Mail Just Fails.
One of the challenges of working in email delivery is knowing enough to be able to separate out the random delivery failures from real delivery issues.

Deliverability • Delivery Improvement