A bug was found in the avast mail scanner. It affects only those mail programs that use a line 'X-Sender:' in headers of outgoing mail (e.g. Eudora). To workaround it, you can disable checking of outgoing mail and keep only checking of incoming mail. Use the Mail Protection Wizard and select 'Set up the protection manually' to do it.
Or you can donload the fixed file here:
http://www2.asw.cz/~peterka/335/ashMaiSv.exe, but the file will be overwritten to the original version during each (automatic) attempt to update the virus base (with a request to restart).
Sorry and thanks for your reports.