Email stuck on server?

[Chalky]

I have a ntlworld.com email account.
I have noticed in the last month I am getting intermittent issues with receiving email.

It started happening again tonight, and despite a mass search I have yet to find a solution.

Some emails come down, so my settings must be ok on Thunderbird.
However, now I have received some mails I seem to have one stuck on the server. I select receive and Thunderbird says "Downloading 1 of 3" but it eventually times out.
I have Outpost Firewall so I can see there is a connection to pop.ispmail.ntl.com but nothing happens.

I have another email account for my partner and she has the same issue. She now has pretty much given up and uses webmail. I don't want to use webmail as I need to keep some emails.

In fact I can sometimes delete one email from my webmail inbox and then it starts working again. It's all very odd?
Any ideas?

---------- Post added 30-11-2009 at 00:25 ---------- Previous post was 29-11-2009 at 23:10 ----------

Ok, I think I've fixed it.
It was Outpost Firewall.
To fix it: I changed the application rule for Thuderbird 2.0.
Set Content Filtering to Disable Filtering
That's been driving me nuts.