Constantly asking for user name and password on every send regardless of account
Windows 10, Outlook 2013, 4 e-mail accounts including AT&T. I can send and receive to the AT&T account but everytime I send an email, regardless of the account, I get a pop-up asking for my AT&T user name and password. It doesn't matter if I check the "save password" box, it continues to ask for the credentials everytime. It also pops up periodically during the day at random times. Sometimes I have to click [Ok] five to eight times in succession before it quits popping up. I am able to send and receive mail from the AT&T account in Outlook.
I've logged on to the ATT.net site, clicked mail and logged in, gone directly to mail.yahoo.com, as I've seen in other posts, but the both redirect to mg.mail.yahoo.com. I can log on without any trouble and send mail.
I've followed the advice of other posts using netsh to reset winsock and ip. I've deleted and recreated the AT&T account in Outlook, changed the password. Nothing seems to work. There are many posts regarding the same issue on this and other forums but no real solutions... at least that I've been able to find. What else can I do to fix this annoying problem?