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FOSS email providers?
(beehaw.org)
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
This community's icon was made by Aaron Schneider, under the CC-BY-NC-SA 4.0 license.
Another vote for Proton Mail! I’ve been using it for years with a custom domain. Keep in mind that your emails are secure and encrypted on PM’s servers, but not necessarily on the servers of your recipient.