nnmail-broken-references-mailers

nnmail-broken-references-mailers is a variable defined in `nnmail.el'.
Its value is

"^X-Mailer:.*\\(Eudora\\|Pegasus\\)"

Documentation:
Header line matching mailer producing bogus References lines.
See `nnmail-ignore-broken-references'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.