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While migrating from Outlook to Thunderbird I encountered the “17 bytes” problem with imported emails that had zip and doc attachments. The solution was to import the mail from Outlook to Outlook Express and then from Outlook Express to Thunderbird.
I dont’ know if this has anything to do with your problem but the “17 bytes” error message rang a bell in my memory when I read your post.
I have had no problems with avast corrupting files when using the Internet Mail scanner.
Sean D