It’s a good question. I think you’re right in that maybe they haven’t been fully clear yet on that. If the encryption at rest is based on PGP or S/MIME, that would likely be the only scenario where the keys could reside on your host if you have your private key there

Page 13 https://blog.thunderbird.net/files/2025/06/Office-Hours-Thunderbird-Pro.pdf

Similar to Proton & Tuta, they’re storing it encrypted at rest. However you really only have full encryption at rest if the sender encrypts their email with your PGP Key or S/MIME Certificate. There’s nothing stopping Proton or any other provider from making a copy of every email not sent using this as it arrives or sends.

Mozilla Thunderbird is about to launch Thundermail, which looks like it will be more open (no bridge to use in email clients), include a free tier eventually, and of course is owned by a nonprofit unlike Proton.