Setting this variable will cause mutt to open a pipe to a command instead of a raw socket. You may be able to use this to set up preauthenticated connections to your IMAP/POP3 server. Example:
tunnel="ssh -q mailhost.net /usr/local/libexec/imapd"
NOTE: For this example to work you must be able to log in to the remote machine without having to enter a password.