since I move to another flat I can only use pop3 with SSL and SMTP with SSL (security reason) with thunderbird 1.5.0.7. Here I have a local network and the university (my ISP) cancel connection using no encryption. I can’t change it.
I notice that avast home did not check emails (incoming and outgoing).
SSL was designed and exists just to stop anyone intercepting it and viewing what is being sent. That is why it is used for transactions such as your online banking.
Once you switch to using a secure connection for your email (the client you use does not matter) then avast or any other antivirus product can no longer scan the mail.
If the mail service you are using advises you that it is already scanning your mail for viruses then you may well decide that is enough.
If you do want avast to scan this mail then you need to arrange to have the secure connection to the mail server terminated inside your computer and then allow the mail to be scanned by avast as it is transferred (inside your computer) to Thunderbird. The best known product to do this is STunnel (it is a free product) and if you search in this forum you can find many references that would help you set it up.