OK. I asked around and found out this:
There seems to be a bug in Outlook Express, in particular in its Base64 decoder. If a text is aligned (in Base64 format) using the '=' characters (as it should be) and a new piece of text is appended (e.g. when a note is inserted into the incomming message), Outlook Express ignores the alignment - so the inserted part "merges" with the previous e-mail, which results in the corruption you can see.
Try the following: open the avast4.ini file and write
PatchBase64=1
into the [MailScanner] section.
With this option, avast! will append the e-mail notes in a different way. Let's see is it helps...
Is this connected in any way with Avast's inability to stamp it's "clear" message into mail that is signed with an X.509 certificate?
