Been there, done that. After a bit of back and forth, Telekom basically recommended that I go and use one of the big SMTP servers and stop bothering them. While I hated myself for doing it, I eventually switched to Gmail for peace of mind.
Does Fastmail have any clout in Europe? I've been a customer for the better part of a decade (with my own domain name) and I've never had a mail delivery issue.
I was going to suggest Fastmail too. I don't know about Europe in particular but have been a very happy Fastmail customer for several years, running mail for 2 small corporations plus personal, zero problems ever.
This is one of the reasons why I'm not planning to host my own e-mail server. It's not that I can't do it, but I don't want to sink time into investigating and working around/solving things like that.
The small boutique mail hosts are also much more tedious to deal with than any of the big players. So it depends on your recipients how much effort self-hosting is.
