-If this is nil while `mail-specify-envelope-from' is non-nil, the
-content of `user-mail-address' is used."
+This only has an effect if `mail-specify-envelope-from' is non-nil.
+The value should be either a string, or the symbol `header' (in
+which case the contents of the \"From\" header of the message
+being sent is used), or nil (in which case the value of
+`user-mail-address' is used)."