Tutanota is similar to protonmail with an encrypted inbox and emails as well can be
Mailpile is good for a self-hosted version
What do you mean by open source email service?
The script might be open source but there is no way in hell you are going to find out what they use one their servers.
As the user above, go for proton mail.
If it's just for private stuff not running a business the free plan is more than enough.
I don't think you're going to use it for sending DWG files or something like that.
The server used for hosting an email service is irrelevant with what the guy is asking. Open source means the code and documentation are open to the public
There are much businesses using protonmail right now with their own domain