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One of my colleagues was using it for ~3 years now and is considering switching, as their anti spam is less and less useful. Some days he wakes up to 30-40 spam/scam emails not being blocked, while he also did miss important mails being filtered as spam. (There is no spam folder on posteo, they just don't deliver spam emails to your inbox. Keep that in mind).

When he tried to forward these mails to the email address they provide for reporting spam, he got an automated mail delivery system email, that he was blocked from sending mails to that address...

I've been with them for a couple years. Also very happy with the service. Quite cheap.

I use it with my own domain and its been stellar.

Do you use a mail forwarding service along with Posteo, since it doesn’t support custom domains on the service?

Yes, in a way. My DNS provider, LuaDNS, has a pseudo-record of type FORWARD [0] which sets up both the MX records and the forwarding.

Other than that, I set up SPF and DMARC.

[0] https://luadns.com/help.html#forward-record

OP wants custom domain support, which Posteo explicitly does not support and will not support directly.