On mailcow page I can see only one provider and it’s Germany based. This seems to me as sub optimal, since DartNode seems to be USA based provider.So I guess you would appreciate something more local to you (no need to transatlantic transfers)
I’m also EU based, I can recommend only one for you. Hetzner it’s strong and reliable Germany (EU) based provider whit multiple location including multiple EU location, 2 in USA and 1 in Asia.
Whit some quirks MC can be run on Hetzner’s VSP (you will must overcome blocking 25 port by time test, also look here about Hetzner)