Outlook not sending mail

I am running Avast 5.1.889

For the past few days, when I try to send mail using Outlook 2000 the mail remains in the Outbox and I get a message saying that the TCP/IP connection was terminated by the SMTP server ( mail.btinternet.com ) Incoming mail is working OK

I am not scanning outbound messages in the Avast Mail Shield.

If I stop the Mail Shield Outlook sends the message.

It looks like a settings problem somewhere - in my Avast? - but I am unable to find where

Can anyone help point me in the right direction please

https://support.avast.com/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=458