What happens if you save the message to a file on disk and scan it with avast (e.g. by right-clicking it in the Explorer and selecting Scan...)? Does it work?
Also, could you send me such a message?
This sounds like something that really shouldn't be hard to fix, if we only had the appropriate samples.
Please send the message untouched, i.e. as an eml attachment (don't just fwd the message).
Thanks,
Vlk