CREATE FUNCTION Fails with an Insert Statement in it

I have created the following function :

CREATE OR REPLACE FUNCTION sp_insert_tbl_l_header(int4,text)
RETURNS bool AS
$BODY$
INSERT INTO tbl_xyz
(unmask_id,email_from)
VALUES ($1,$2)
$BODY$
LANGUAGE 'sql' VOLATILE;

when i try to create this fucntion by running this script, i get the
following error:

ERROR: return type mismatch in function declared to return boolean
DETAIL: Function's final statement must be a SELECT.
CONTEXT: SQL function "sp_insert_tbl_xyz"

So i think there is some problem with the return type of the function
when it has an insert statement but really don't know what should be
the return type of this function.

Thanks,
Jas