mail-complete-alist

mail-complete-alist is a variable defined in `mailalias.el'.
Its value is shown below.

• This variable may be risky if used as a file-local variable.

Documentation:
Alist of header field and expression to return alist for completion.
The expression may reference the variable `pattern'
which will hold the string being completed.
If not on matching header, `mail-complete-function' gets called instead.

You can customize this variable.

Value:

(("^\\(Resent-\\)?\\(To\\|From\\|CC\\|BCC\\|Reply-to\\):" mail-get-names pattern)
 ("Newsgroups:" if
  (boundp 'gnus-active-hashtb)
  gnus-active-hashtb
  (if
      (boundp news-group-article-assoc)
      news-group-article-assoc))
 ("Followup-To:" mail-sentto-newsgroups))