fcc-hook [!]pattern mailbox
This command is used to save outgoing mail in a mailbox other than $record. Mutt searches the initial list of message recipients for the first matching regexp and uses mailbox as the default Fcc: mailbox. If no match is found the message will be saved to $record mailbox.
See Message Matching in Hooks for information on the exact format of pattern.
fcc-hook aol.com$ +spammers
The above will save a copy of all messages going to the aol.com domain to the `+spammers' mailbox by default. Also see the fcc-save-hook command.