Not directly understood by Procmail, but you can get an external program and use that do deliver to MBX. Used by some IMAP systems (?
Ian McNish writes: Procmail can write to mbx formatted mail folders via the ‘dmail’ utility. dmail is included in the UW imap-util distribution. Usage: DELIVER_LOCAL_MAILER = /local/bin/dmail MAILDIR = $HOME/.Mail DEFAULT = $MAILDIR/inbox :0 w | $DELIVER_LOCAL_MAILER +inbox Thanks for this information! (More details still welcome.