Jorgo, did you try to increase both timeouts (of avast into the Advanced tab of Internet Mail provider settings and of your email account, in Properties of the account, Advanced tab too). You can set both to 600, for instance. If you’re on a DSL connection (not dial-up) it shouldn’t be necessary a so long timeout anyway…
The message “Potential infection detected” is due to your Heuristic Mail settings.
Reinstalling avast won’t solve, of course, but you can uncheck the Outbound mail scanning.
Hope this helps.