It's very difficult to get it all correct enough to keep a significant portion of the email sent through your SMTP site from being considered spam by mainstream

It's very difficult to get it all correct enough to keep a significant portion of the email sent through your SMTP site from being considered spam by mainstream email providers such as Google. This doesn't change the problems of unreliable email because mail services are too aggressive, unfortunately. We recently got a bounced mail (not dumped in a spam folder, actively rejected) from a major university, telling us that the message looked like unsolicited bulk mail. The message was sent directly to a specific single customer in response to a purchase they had just made, contained information that we were required by law to provide to them, and was sent from a reputable host with things like SPF properly set up. That is simply broken, and it is 100% the fault of the mail service admins at the university.