I've had no issues with it except for one hostel I tried to email not getting my email. The web interface isn't the best, but I access my email almost exclusively though IMAP. I've also set up my calendar and contacts though it.
I picked posteo over proton and tutamail because of the IMAP support. I didn't want to use a proprietary app.
I basically need to stick this xkcd chart next to my computer or I'll spend days creating something that saves me minutes per years.
Although sometimes it's fun to automate things just for the hell of it.