Sorry, I kind of forgot about lemmy or a few days. In Thunderbird, I create a new dedicated folder, use Tools --> Message Filters. I then can add the desired filter (something like must contain at least ‘host.tld’ in sender) and make it move all filtered mails into the previously created folder. I just checked, it works like this. You can also specify when that filter should be executed (e.g. when getting new mails or every 10 minutes) and the folder with the filtered mails also shows up in FairMail on Android. Better description: https://support.mozilla.org/en-US/kb/organize-your-messages-using-filters
There is encryption at rest (storage encryption), transport encryption and end-to-end encryption. E.g. Posteo has transport encryption and optional storage encryption. With activated storage encryption, Posteo cannot read your mail because the encryption key on their server is only usable with your password (which they do not store). Proton Bridge adds end-to-end encryption to Protonmail