Actually, I have the same problem, i've managed it temporarily by making the .bat file, which manages the return codes from antivirus and returns what mailscan expects (that's because DrWeb returns plenty much of info and uses all 255 rc's)
Startup info like this:
24.02.2004 10:23:01 Mailscan: In MainEntryPoint() - Registered
24.02.2004 10:23:01 MailScan: Startup……
24.02.2004 10:23:01 MailScan Configuration: Aufruf des Scanners mit "d:\bin\drweb\DRWEBWCL.EXE"
24.02.2004 10:23:01 MailScan Configuration: Aufrufparameter "/INI:D:\bin\drweb\drweb32.ini [Name der Datei]"
24.02.2004 10:23:01 MailScan Configuration: ReturnCode0: 0 - Text: ""
24.02.2004 10:23:01 MailScan Configuration: ReturnCode3: 3 - Text: "(VIRUS FOUND)"
24.02.2004 10:23:01 MailScan Configuration: ReturnCode8: 8 - Text: "(VIRUS FOUND IN ARCHIVE)"
24.02.2004 10:23:01 MailScan Configuration: Error EncryptedMail: "(NOT SCANNED)"
24.02.2004 10:23:02 MailScan Configuration: Other Error: "(ERROR OCCURED)"
24.02.2004 10:23:02 MailScan: Initialization complete.
24.02.2004 10:23:02 MailScan: Searching for Mailboxes.
24.02.2004 10:23:02 MailScan: No Multiple Mailboxes.
24.02.2004 10:23:02 MailScan: Startup Completed.
and after scan reported an unknown error.