Authenticating with AT&T

Similar to Yahoo and AOL, AT&T also requires that an App Password (or as AT&T calls it, a Secure Mail Key) be used to authenticate instead of the target account's main password.

You can create a Secure Mail Key as follows:

https://www.att.com/support/article/email-support/KM1240308/

Note: It is important to follow the Profile link in the above article (repeated below) as the Profile menu may be unreachable through AT&T's webmail interface.

https://m.att.com/myatt/native/deepLink.html?action=Profile&appInstall=N

Once you follow the above link and authenticate, you should find the Manage Secure Email Keys menu toward the bottom of the page.