I wish. I've been having issues with being sent to spam when replying to a colleague on the same domain which is on Google. The email never left their servers, everybody was authenticated, and the emails were within an hour of each other, so it's pretty obvious that it's a reply. Still: off to spam it went.

We now have a filter rule to always consider our own domain as not spam, because it's just too unreliable otherwise.

If you get an incoming email “from” yourself and it is not marked as spam, it will be put into your sent label.

At least one person claims to have been incorrectly fired because of this. Their employer found incriminating emails in the person’s sent box, and considered it a closed case.

In that time I've had a single incident (earlier this year) where spammers got it and sent 20k spam mails in less that 24h mails. Luckily I noticed this and stopped it pretty quickly.

I had mails to some destinations bounce for around a week after that.

Beyond that incident, it's very rare to have trouble delivering mail - I think maybe once in the past 5 years there was a company who I had trouble with.

Having said all that, the spam incident this year highlighted how fragile hosting your own mail server can be if things go wrong, and I felt really bad about the spam mails sent from my server - continuing to run it almost feels like a liability, and when time permits I plan to move our mailboxes to O365

Hmm, I just tried what you suggested, with 2 different accounts never sent to, and the emails got through fine.

We use SPF and DKIM, but otherwise it's a fairly standard Postfix/Dovecot install (albeit a very old one).

I have a pet theory that big email platforms care deeply about the IP "neighborhood" the sending email server lives in. So a small email server set up perfectly in Digital Ocean IP space would struggle, while an email server set up OK in (for example) CenturyLink Enterprise IP space would get through.