-a headerfield
Append a custom headerfield onto the header; but only
if a similar field does not exist yet. If you spec-
ify either one of the field names Message-ID: or
Resent-Message-ID: with no field contents, then for-
mail will generate a unique message-ID for you.
-A headerfield
Append a custom headerfield onto the header in any
case.
Cheers, Rob