Yarn checks if the original message was sent to other recipients
as well as to yourself. It does this by looking in the To and Cc
headers for e-mail addresses that don't match you own e-mail address.
If that's the case, the Yarn reply function asks if you want to
send your reply to only the author, or send your reply to the author
and everyone who received the original message. If the original
message had a Reply-To header, Yarn gives you an additional option
of sending your reply to only the Reply-To address.
Messages from the Yarn mailing list have the e-mail address
yarn-list@listproc.colorado.edu in the To header. Since this
address is different from your own e-mail address, Yarn knows
the message was sent to other recipients. When you tell the
Yarn reply function to send your reply to all recipients, that's
one of the addresses where your reply is sent.
If you filter the Yarn mailing list into a pseudo-newgroup as in
the example in the english.doc manual, using the follow-up command
inside the pseudo-newsgroup will send your follow-up to the mailing
list address.