> I guess Google's customers are just more trustworthy...

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.

Note that your filter might be breaking an important security feature:

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.

That's interesting, do you have a link for that case? I don't see why it would get put in the Sent folder, but if it does, that might certainly lead to confusion.

This is a really annoying Gmail+Gsuite bug that has affected us as well. Surprising that it seems to happen this often.

Counterpoint. I run my own mail server, and have done for 10+ years on the same IP.

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

Unless you have substantial volume to gmail (hundreds of mails a day), I can virtually guarantee gmail is spam boxing your mails, try mailing an account you have never sent mail before for instance.

They don't refuse delivery, but that is entirely meaningless when nobody ever checks the Spam box anyways (which is also hidden by default)

Our email volume is pretty low - average is probably 5-10 a day.

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).

Would you mind sharing where you host your email server?

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.

It's with a European (Austria-based?) VPS provider, Edis (edis.at), and my server is hosted in a UK DC.