mails are not going out side of my domain

hii ,

I am a system engineer in my organisation and i have exchage 2003 with sp2 , the scenario is that i have dc in windows 2003 (sp1) both dc and exchange 2003 are on same dell mid range machine half of my harddisk is empty because it is recmonded from microsoft , well i have an smart host on another machine . and then i have a pix then router , first of all mail is went to smart host then pix and then router and router will send it to outer world from few days i am getting an problem that my mails are not going out side i have checked queue of system manager its shown an blue color of arrow it means that when i am sending the mails to another domain it is not able to find that domain no error message is occur in event viewer it means my mails are stuck in queue but its not showing any message please help me regarding this matter that what will be the problem.

Regards Muhammad Ali