8.6.1. Per-Recipient Rules Editor

To edit per-recipient rules, select OpenPGP → Edit Per-Recipient Rules.
The picture below shows the Per-Recipient Rules Editor window with one created rule.

I participate in the mailing list for the developers of Enigmail and, as a habit, all subscribers always sign and encrypt all messages to the mailing list. A mailing list is just a list of physical recipients and as such does not own a key pair, as this would made no sense. Therefore, to encrypt a message for a mailing list, I have to encrypt it with the public keys of all people that subscribed to the mailing list (of course, I need to know who the subscribers are, and I need to have their public keys).
For this purpose, I wrote the above rule, which says: “When sending a message to the address specified in Email, always sign it, always encrypt it with all the public keys specified in OpenPGP Key(s), and never use PGP/MIME”.

If you create more than one rule, they are evaluated in order from top to bottom. You can change the rules order by using the buttons Move Up and Move Down, while Delete will delete a rule.
The Add button adds a new rule, and Modify modifies an existing rule. Clicking these buttons will bring you to the Recipient Settings window.