alanrf
10
Just one extra thought.
I know it is unclear whether a program update or just a VPS update occurred earlier today. The avast logs should have the details.
I noticed in the TB forum you mentioned clearing the Thunderbird entries in the ZoneAlarm program table and letting permission be asked again since you are aware that ZoneAlarm can sometimes become confused after a program update.
When using the avast Internet Mail scanner it is equally important to perform the same step for ashMaiSv.exe (the process name for the avast Internet Mail scanner) which is the process that actually initiates the connection to the real mail server. If it were being blocked by ZoneAlarm after an update it could certainly give rise to the symptoms you saw today.