cant send email from exchange server to yahoo

hello brothers
we are using exchange server 2003 and we are using vigor2800 router everything is working fine.we can send and recive emails from out side.i open smtp 25 on router.everything is sounds good.now the problem is we cant send emails to only yahoo tho we can recive email from yahoo.i cant understand what this problem is.
could any body tell me solution of this this problem.plz