I'm using the latest version of Avast Free Antivirus (build 17.9.3761.0) on Windows 10. Thunderbird stopped sending email about a week ago with the following message:
"An error occurred while sending mail: The mail server sent an incorrect greeting: Cannot connect to SMTP server 2604:4f00::1:0:0:182 (2604:4f00::1:0:0:182:587), connect error 10061."
It also wouldn't receive email but there was no visible error message for that.
The problem was circumvented by disabling Avast Mail Shield.
I was told that the mail server in the above error was using an IPv6 address. So I tested IPv6 connectivity on my network and found that it was broken.
I disabled IPv6 on my network (Network and Sharing Centre > Connections: connection name > Properties), enabled Avast Mail Shield, and the problem has disappeared.
So I have two questions:
1. Is the email really being checked by Mail Shield now, or has disabling IPv6 on the network had the same effect as bypassing Mail Shield?
2. Why doesn't Mail Shield fall back to an IPv4 connection when the IPv6 connection isn't working?
Program versions:
Avast: version 17.9.2322 build 17.9.3761.0
Windows 10 Home: version 1709 build 16299.125
Thunderbird: version 52.5.0 (32-bit)
Thanks,
Ian
