I found Tuta to be lacking.
Conversation view is incomplete https://github.com/tutao/tutanota/issues/6 - https://github.com/tutao/tutanota/issues/5051
“when you have multiple addresses and custom domains getting hundreds of emails… it takes forever for the emails to load” https://community.centminmod.com/threads/skiff-email.24363/
Search isn’t working in firefox “your browser doesn’t support data storage”. As the search index needs to be stored in your browser, it does not work in private mode/incognito mode.
Free accounts get deleted if you do not log in for six months.
I ran into a similar problem with snapshots of a forum and email server – if there are scheduled emails when you take the snapshot they get sent out again if you create a new test server from the snapshot. And similarly for the forum.
I’m not sure what the solution is either. The emails are sent via an SMTP so it’s not as simple as disabling email (ports, firewall, etc.) on the new test server.