unable to send email - blocked

I am unable to send email. My server states they are unable to help, that it probably a virus/trojan affecting my system. I have been running Avast for a while and never had this problem before. Can you help. I utilize Outlook Express/ WindowsXP SP2. Incoming mail is OK it is only the outgoing that is affected.

Does sending mail work if you disable the ‘Internet post’ provider in Avast?

When you say your emails are blocked, what error messages are you getting to indicate this?