I have an active Avast free anti-virus (program build 12.3.3154.0, definitions 160822-1) installed on a laptop running Windows 10 Pro X64.
Recently, I could not receive or send e-mails in Thunderbird 45.2.0 any more. No changes to the installation, except for the automatic updates of Windows & Avast. I update Thunderbird manually after an alert of a new version. It seems that the problem must be caused by an automatic update that changed a setting on the PC or a virus.
I then checked for malware on the system despite active shields in place. I ran a full scan and also used another malware scanner to scan the computer. No problems reported. I then suspected Thunderbird. I uninstalled it and download a fresh copy and install it. I could not set up the accounts as Thunderbird's wizard reported problems when verifying the user name and password for GMail and Yahoo accounts.
After I installed Opera Mail and landed with the same problem, I realise that the problem is most likely with the firewall blocking access.
After installing Thunderbird again, I disabled the mail shield in Avast for 10 minutes and tried to set up the accounts again. It worked fine. I could send and receive messages. When I switch the mail shield in Avast on again, no e-mails in or out.
I added Thunderbird to the allowed programs/apps in Windows firewall and I checked that the Avast SSL certificate appears in the certificate list of Thunderbird. I also verified that ports 993 and 465 are in the list of mail ports in Avast troubleshooting section. This did not solve the problem.
The only options I currently seem to have is:
1. Turn mail shield of permanently - not acceptable
2. Get another anti-virus - I have the same setup running on my desktop PC with another anti-virus and there are no mail problems.
Any suggestions?