Section 4.7 of mutt manual:
4.7. Mailbox Shortcuts
There are a number of built in shortcuts which refer to specific
mailboxes. These shortcuts can be used anywhere you are prompted
for a file or mailbox path.
* ! -- refers to your $spool'' (incoming) mailbox
* > -- refers to your$mbox'' file
* < -- refers to your $record'' file
* - or !! -- refers to the file you've last visited
* ~ -- refers to your home directory
* = or + -- refers to your$folder'' directory
* @alias -- refers to the ``default save folder''as determined by
the address of the alias