Three possible scenarios against "vendor lockin" in my opinion
use fetchmail and dovecot as described better by the other posts here
use imap on your third party and make regular backups using offlineimap (if using linux...). Tools like this should exist for other OSes too
use imap on your third party. When you are no longer satisfied get yourself a new party, and transfer the whole shebang with imapsync. As long as the mails are accessible via IMAP, youre good to go.
Myself i'm using offlineimap as a systemd-timer in background continously and imapsync last time i switched from my own mailserver to a third party solution.
Both work like a charm.
Three possible scenarios against "vendor lockin" in my opinion
Myself i'm using offlineimap as a systemd-timer in background continously and imapsync last time i switched from my own mailserver to a third party solution. Both work like a charm.