Not getting external e mails

Hi,We use exchange 2003 as our company e mail server.It is running on a Windows 2003 server. Everything works fine apart from the fact that every 3 months or so it stops receiving external e mails. We can send externally but cannot receive. We can also send and receive internally no problem. A restart of the server seems to solve it but can anyone tell me what might be going on and how we could cheak and fix without having to restart the server. Thanks.