That facility does not exist in the Pegasus address book (though it is in other mail clients). However, you can do it through other means. This is what I do:

If you filter messages from your mailing list into a separate folder, you can then set that folder to use a different Pegasus Identity when creating replies to those messages (right-click on the folder and choose "Set default identity for entry"). If you make sure that the identity you choose is configured to only ever send plain text messages (Tools > Options > Message Formatting > Disable all text styling options), everything will be fine.

With this method you can also exploit the other benefits of an alternative identity such as the From name being something different or perhaps sending through a different server.

Some of my mailing lists will only accept emails that have been formatted in TEXT mode only.

Is there any way to mark these few  addresses in the Address Book so that they

are always sent in text mode.

John.

Have you tried sending all the messages with the "Generate multipart/alternative versions of richtext messages"

checked in TOOLS-->Options-->Outgoing Mail-->Sending Mail? That might provide the receiving e-mail client

the option to read the text-only version.

 Dave Bellerophon