Prio High as this was a promised feature in 3.1.0 (improved Automatic resolution).
If Autoresolve is active it should fall back to S/MIME if there are no OpenPGP Keys for all recipients with at least the validity marginal. (And it has S/MIME Keys for all recipients instead).
Originally reported here: https://wald.intevation.org/forum/message.php?msg_id=5877&group_id=11