mail-abbrev-mode-regexp

mail-abbrev-mode-regexp is a variable defined in `mailabbrev.el'.
Its value is

"^\\(Resent-\\)?\\(To\\|From\\|CC\\|BCC\\|Reply-to\\):"

Documentation:
Regexp matching mail headers in which mail abbrevs should be expanded.
This string will be handed to `looking-at' with point at the beginning
of the current line; if it matches, abbrev mode will be turned on, otherwise
it will be turned off. (You don't need to worry about continuation lines.)
This should be set to match those mail fields in which you want abbreviations
turned on.

You can customize this variable.