Until yesterday all was fine with my Virgin email account, which I have habitually accessed via "Mozilla Thunderbird" on my desktop PC.
Yesterday I was told by Thunderbird that the certificate issued by Virgin Media was not trusted. At first I ignored this and sent out an email. Shortly afterwards, I received an email from an unrecognised domain that climed to have been sent by me! I did NOT click on the link that this email contained.
I then moved my profile directory to a hidden location on my PC and uninstalled Thunderbird.
On reinstalling Thunderbird, I was told that my profile was not available; but was NOT given the option of setting up a new profile. Thunderbird simply terminated. When I put my Thunderbird profile directory back where it belonged, I was told again that the certificate from Virgin Media was not to be trusted.
I then tried installing Thunderbird on my laptop. This was straigntforward, and I was able to set up a new profile for a secondary Virgin Media email address - I was NOT given this option when reinstalling Thunderbird on my desktop PC. However when I tried contacting Virgin Media via Thunderbird, I was once more told that the certificate supplied was not trusted.
Given this experience with my Lap-Top and a different email address, I conclude that the problem lies with Virgin Media itself, as there is no other common factor. The problem arises independently of which PC I use and which email address I attempt to access: only the email provider is in common.
In order to do SSL scanning it actually acts in the same way as a MITM attacker. When you connect to the mail server, the connection is directed to a socket on your PC instead that avast is listening on. It then goes and makes the outbound connection to the mail server separately. In that manner it's able to scan your mail as it arrives from the server.
If the Avast certificate expires then that can cause certificate errors.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.