I don't know what your ISP or what your email client are, but mine, BT Internet, has Yahoo handle my email, but I don't have yahoo in my email client account settings, I enter my ISP email server details mail.btinternet.com and usual ports and that is somehow handed off to yahoo.
But I don't see that as it happens in the background, only when I use webmail do I then see the BT Yahoo branding.
So there is certainly something weird going on with your ISP and email server settings, never seen anything like this before though. I would start from scratch and visit your ISP site and confirm that your email settings are correct.
But if those two entries are still in the avast SSL settings I would remove them, you didn't say if you had done that or not.