mail-citation-prefix-regexp

mail-citation-prefix-regexp is a variable defined in `sendmail.el'.
Its value is

"\\([ 	]*\\(\\w\\|[_.]\\)+>+\\|[ 	]*[]>|]\\)+"

Documentation:
Regular expression to match a citation prefix plus whitespace.
It should match whatever sort of citation prefixes you want to handle,
with whitespace before and after; it should also match just whitespace.
The default value matches citations like `foo-bar>' plus whitespace.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.