I would like to REALLY archive some older e-mail-folders. Archive seems not to be the right word, because i work with empty inboxes, so every completed or answered e-mail is still archived in a folder (with the name of the year), but i use IMAP so all archived emails are still on the server.
Now i want to EXPORT the years 2018-2019 to my selfhosted-backup-server so that i can delete the emails on the mail-server.
a) Which format make most sense? EML? mbox? Another one? Would be nice if i can search the archived stuff without too many effort. b) Should i export directly from the UI of my mailprovider (if possible) or is exporting from thunderbird-client ok.
You could use mbsync to automate this. You can configure it so that it downloads new mail with IMAP, but keeps them even if they are deleted from the server.
Then you will have a Maildir that you can manage with any mail client, index with notmutch, etc.