The answer Yes in Silent Mode keeps the virus in the file or into the message (attach) and continue the scanning. You can’t configure ‘delete the infected file’ in the Home version. The default answer No will send the file (email) to Chest.

Can´t you increase these timeout to the limit of 600 seconds?

The solution is simple. You must exclude the MailWasher process from the automatic virus scan.
Find your avast4.ini (generally C:\Program files\Alwil Software\avast4\Data\avast4.ini) and in the section [MailScanner] add (or edit) the following key:

[MailScanner]
IgnoreProcess=MailWasher.exe

Multiple process names can be separated with commas.

This way, not the executable will be on the exclusion list but only the mail communication of the ignored process will be bypassed of scanning. This brings no other exceptions - file access is of course scanned. In version 4.5, with the transparent mail scanner of avast!, not only mail clients could be monitored but also all communication to certain ports.

If you believe in MailWasher (of course!) and you exclude it from scanning, no mail scanning on it’s connection will ever have any other effect that timeout message. The timeout message is there for the situations when sending or receiving of large mail may look like timeout and you would like to wait a bit longer in stead of just silently terminate such a connection.