Many thanks for your reply.
Just a bit puzzled, are you hinting that there is a problem with Thunderbird and slow mail checking? Certainly there is on my PC, but is this more general, is it linked to Avast?
I'm saying that Standard Shield could have a slow TB files scanning.
I'm NOT saying that the Internet Mail provider scanning of TB is slow.
The work around you suggest doesn't it leave the mail folders vulnerable to viruses?
No, I'm suggesting you remove the scanning from Standard Shield.
The mail will be scanned, when downloaded (or sended), by the Internet Mail provider.
Oh, sorry, I'm not an expert on mail problems. Maybe Alan or Vojtech could help you here...