Thunderbird CAN work perfectly with Thunderbird, but the process is a bit complicated compared to other email clients. I remember the process only in general terms.
- You must create an avast! certificate using avast! itself, the button is somewhere in the email UI
- You must add the certificate in Thunderbird... can't remember where the option is, somewhere in Security and Certificates.
Done!
Sorry but, I repeat, cannot remember the process in detail.