Exchange Emails not being sent

Hello everyone. This is my first post on here so please bear with me 🙂

I have recently taken on the administration of our servers at work. We use Exchange Server 2003 for our emails. Today we have noticed that our emails are not being sent out. We are still receiving emails ok.

Can anybody help me in troubleshooting this as I have no experience of Exchange Server administration.

Kind regards

Steve Williams