If you send an PGP encrypted mail to multiple receivers, it would be pretty convenient to ask (not fully automatically) the user if he wants to attach all receivers public PGP keys. Most users probably won't think of this problem in the first place, until someone complains that he doesn't has all the other's public keys when trying to "reply to all". Guess this would be a great step forward to enable even less experienced users to use PGP for mails in groups with multiple receivers. Exception: When receivers are BCC, their keys probably shouldn't be attached. Reproducible: Always