alanrf
3
I am using Thunderbird 1.5.0.5 also with 2 mail accounts on mail.comcast.net being scanned by the avast mail scanner (among quite a few others) - almost completely without incident.
However, I also get exactly the error that you posted in the TB forum from time to time.
Everytime I had that message (and bothered to check) logging onto Comcast.net via the web interface and trying to get my mail gave me the error message that the mailbox was not available and sorry for the inconvenience. Sure enough trying again later always finds it working.
In the instances I have checked I have never been able to see any issue with Thunderbird or with avast - it has always been a Comcast outage.