I have a SMIME certificate at work to digitally sign my mails. I don't use it all the time as it confuses a lot of people. ( Perhaps I should it should really be a more commonly used tool). I need to use it for an automated services but have forgot a couple of times so I have just added some mutt, my current mail client of choice, setup to do this for me. If you want to setup mutt to autosign mail only for a select group of people but make sure that it is done all the time. Then you can use a send-hook to do it:

send-hook . "set crypt_autosign = no"
send-hook '~t [email protected] |~t [email protected]' "set crypt_autosign = yes"

The first hook sets the default to no auto sign, the second search the to address, (~t), for fred or ( "|" ) or jim and sets it to auto sign on that mail.

Send hooks are really cool you can change any setting that you like in the second part and search for any part of the email before setting it.