outlook 2010 failing to send an receive emails

Hi

When attempting to send any emails from one of our account we recieve an email stating that the message was undeliverable. The worrying thing is that today is the first email we have received about this but is from an email i sent over a week ago.

I have noticed that no emails are coming into this account either and am beining to wonder if any of the emails i have sent in the past week have actually sent as this is normally an extremely busy email account.

I have checked that I have not reached the data limit for sending or receiving emails. I have also compared this email accounts settings to others on the exchange server but cannot see anything that is causing the emails to send

Any ideas?