AT&T uses Yahoo Mail servers for their email accounts. Since Yahoo requires secure connection when using external apps, that means you will need to use oAuth or a secure App Password to set up your account in external apps.
App Password for ATT accounts
AT&T calles their app specific passwords a Secure Mail Key and you can create one on this webpage:
Create a secure mail key.
Since this link leads to the official ATT Support web you will get the most current information on how to create and use your new secure password.
AT&T accounts in eM Client
Once you have your Mail Key ready it is very easy to set up your account in eM Client.
Simply go to Menu > Accounts, click + Add account and insert your ATT email address. Once the text field for Password shows up, insert your Secure Mail Key instead!
Then just follow the instructions to customize your account name and other details in eM Client as for any other account.