I've been using Cloudron Mail for a long time with 3 different providers (Linode, OnetSolutions and Hostinger). I never had an issue on sending and receiving e-mails.
What bugs me a lot is the search. It doesn't work well even with full-text search enabled...
Another issue is the lack of good webmail software to use with Cloudron Mail server. Any other mail provider (Gmail, Proton etc) have a much better UX than what is available on Cloudron. I used for some time SOGo but now I use Nextcloud Mail and Snappymail's Nextcloud app as a fallback. From now and then I ask Mozilla to develop a Thunderbird Web