Mac mail can't verify password

My mother (who's having me help with some tech issues) has an iMac running OS X Yosemite. I had to reset her Yahoo/att email password, but when prompted to enter the account password in Mac mail, I got the "cannot verify password" message. I tried to manually type in the new password in Internet Accounts under system preferences, in settings > accounts on the mail app, and in the SMTP server list. I still kept getting prompted to sign in, and getting the same message. I eventually deleted the account from both Internet Accounts and the mail app, restarted the computer, and tried adding the account, but I still got the same message. In the mail app settings, under account information, under "Outgoing Mail Server (SMTP) it lists "att.net (Offline)," but on the main mail page the server is listed as online (I tried taking it off- and online there, which had no affect). Does anyone have a solution?