mh-mail-header-separator

mh-mail-header-separator is a variable defined in `mh-e.el'.
Its value is

"--------"

Documentation:
*Line used by MH to separate headers from text in messages being composed.

This variable should not be used directly in programs. Programs
should use `mail-header-separator' instead.
`mail-header-separator' is initialized to
`mh-mail-header-separator' in `mh-letter-mode'; in other
contexts, you may have to perform this initialization yourself.

Do not make this a regular expression as it may be the argument
to `insert' and it is passed through `regexp-quote' before being
used by functions like `re-search-forward'.