When mutt suggests a default save destination, it uses the
"username" part of the e-mail address. Often these names
contain '.' characters. Courier IMAP server uses '.' as a path
separator, so allowing them in mailbox names doesn't work well.
Here's a small patch that converts any '.' characters in a
default save mailbox name to underscores:

http://www.visi.com/~grante/muttcourierpatch.html

--
Grant Edwards grante Yow! FEELINGS are
at cascading over me!!!
visi.com