Please have a look here on the Thunderbird/AV issue:
http://kb.mozillazine.org/Download_each_e-mail_to_a_separate_file_before_adding_to_Inbox
Fast
Fast,
I reported on this feature of Thunderbird 1.5, at length, here in the avast forum and in the Thunderbird forum. Igor went to some length to explain why avast could not scan these files created by Thunderbird.
This really has nothing whatsoever to do with the issue at hand - the avast Internet Mail Scanner does an excellent job of keeping viruses out of the Thunderbird mailbox in the first place. Should a virus get in then we have the backup of the Standard Shield to prevent a bad file from being executed. That’s why I strongly recommend the use of both providers to users of Thunderbird.
The real issue here is avast’s current inability in the on demand scanners to recognize a Thunderbird mail file and the inappropriate actions it takes on the file because it thinks that a Thunderbird mail file is really a single standalone EML file.
The problem here is most likely going to be affecting a new avast user. The first time they run an on demand scan of their system they may well find under, under certain conditions, that avast has destroyed their Thunderbird Inbox containing all of their email received over months or years. As we saw at the start of this thread - it happened to the originator - who had no backup.