Hi,
the avast found a virus in the Panda ActiveScan folder!?
I test this file in the online scanners and they didn’t find nothing…
The Chest can’t sent the file!!! It frezzes a minutes and show this dialog. I have set all the settings correctly… 
http://img69.exs.cx/img69/1337/avast_chest_error.png
File with “virus: Win32:Kuang2” - imscan.dll
If you search this board, you find a lot of threads on “kuang”.
Additional info can be found e.g. here.
The error seems strange, however - are you sure the mail settings is OK?
Could be another false detection.
Edit: as igor has pointed out 
http://img38.exs.cx/img38/5268/avast_chest_settings.png
http://img38.exs.cx/img38/240/avast_chest_settings2.png
This is the SMTP that I use to sent e-mails in my E-mail client.
Panda does not encrypt their signature files >:(
They will always be detected by other antivirus… :-\
It’s not an avast problem. It’s a well-known trouble of Panda.
Anyway, sometimes I have trouble to use the Chest mail either…
I can’t find a solution because the problem does not occur all the times.