The person that controls the blocked IP address needs to get it removed from the blocked list. This could be your ISP or email provider. Saying that web mail works is meaningless. This is 2 different operations using different servers. It is sad that your providers support group is not intelligent enough to know this. It is not a thunderbird problem. The IP needs to be cleared from the spam list. Stop deleting things on your local system. You are wasting your time.

Attached is the full error. Just been in contact with my provider and I've actually got a dynamic IP address so it should change after the box has been switched off.

Still no further forward.

When you follow the instructions in the error message this is what you see.

Important: If you are using any normal email software (such as Outlook, Entourage, Thunderbird, Apple Mail, etc.) and you are being blocked by this Spamhaus PBL listing when you try to send email, the reason is simply that you need to turn on "SMTP Authentication" in your email program settings. For help with SMTP Authentication or ways to quickly fix this problem

If this does not fix the issue your need to have the owners of the IP address get it fixed. The IP address comes back to Sky Broadband in GB.

Here is the link to the Spamhaus page. http://www.spamhaus.org/pbl/query/PBL1550975