How can I set reply-to field conditionally?

["Bo Peng" <ben.bob@xxxxxxxxx>]

Dear list,

Because my mail filter sometimes filters out important emails
(although I will periodically review my junk mailbox), I would like to
use an alias account that will not be filtered. That is to say, for
important email I send out and anticipate a reply, I will set the
reply-to field to this unfiltered account. Otherwise, reply-to the old
email account.

I have looked up the mutt manual and the only relevant part is
$REPLYTO. How can I set up a reply hook for this purpose?

Many thanks in advance.
Bo