exchange server 2003 is not reciving emails

hi we just instaled a new vigor2800 router and we set up our static ip address on tht now internet is working fine..
but we are facing some problems regarding exchange server tho we can send emails but we are not reciving any email from out side.
could any body tell me what is the problem..
i set smtp on 25 port but it still not working fine.
could any body help me plz...