message-unix-mail-delimiter

message-unix-mail-delimiter is a variable defined in `message.el'.
Its value is

"From \\([^-\n-
].*\\)?\\([^-
 ]+\\) +\\([^-
 ]+\\) +\\([0-3]?[0-9]\\) +\\([0-2][0-9]:[0-5][0-9]\\(:[0-6][0-9]\\)?\\) *\\([A-Z]?[A-Z]?[A-Z][A-Z]\\( DST\\)?\\|[-+]?[0-9][0-9][0-9][0-9]\\|\\) * \\([0-9][0-9]+\\) *\\([A-Z]?[A-Z]?[A-Z][A-Z]\\( DST\\)?\\|[-+]?[0-9][0-9][0-9][0-9]\\|\\) *\\(remote from .*\\)?\n"

Documentation:
Regexp matching the delimiter of messages in UNIX mail format.