Email, OWA problems (Smoothwall Firewall & Virgin ISP)

[Archipelego1]

Since Wednesday, we have been having problems with receiving and delivering email. We find that emails will stick in the exchange queue for hours, before working through and being delivered.

 

We've also noticed that whenever emails aren't delivered, we're also not able to access the OWA website externally... Although we can can access internally. When we're unable to access OWA, I can still ping our external IP address.

 

Does anyone having any ideas what the problem could be?

 

Nothing was changed prior to the problem. We have a smoothwall firewall and virgin internet connection. Smoothwall weren't able to spot any issues.