I’ve tried setting the protection module disabling SMTP scanning, but without success.
Uncheck the option to scan SSL connections without success.
I exported the certificate smtp.gmail.com.crt without success.
Update to the latest version of avast and check if the problem is solved:
https://forum.avast.com/index.php?topic=166576.0
don’t work.
Have you followed these instructions ?
https://forum.avast.com/index.php?topic=161298.msg1155802#msg1155802