If set to a value greater than 0, Mutt will display which message it is currently on when reading a mailbox. The message is printed after read_inc messages have been read (e.g., if set to 25, Mutt will print a message when it reads message 25, and then again when it gets to message 50). This variable is meant to indicate progress when reading large mailboxes which may take some time. When set to 0, only a single message will appear before the reading the mailbox.
Also see the `` $write_inc'' variable.