Adohall, welcome...
Can you see in the headers of the messages (Right click the message, choose Properties and Details), for instance:
X-Antivirus: avast! (VPS 0XXX-X, XX/XX/2004), Inbound message
X-Antivirus-Status: Clean
1. Configure your email accounts to work without the antivirus and test them.
2. If everything is fine, close all your email clients (programs).
3. Start Menu > All programs > avast antivirus > Mail Protection Wizard
4. Follow the non Outlook/Exchange option (i.e., the second one).
You can choose the supported email programs and select all the accounts
(even the ones created in the future).
5. Configure your email accounts to use avast! as antivirus.