Currently it is possible to add an AT&T based email address to Spamdrain using oauth2 standard where you will just need to authorise Spamdrain to access your email address with no need to enter the password. Once you enter an AT&T based email address during sign up / adding email address you will see the prompt on how to proceed. Please follow the steps in order to authorise Spamdrain.
In case you are still experiencing issue with your mail when Spamdrain can not connect to your email server, you could use the app specific password solution. In order to do that please create a Secure Mail Key for Spamdrain.
You do this in your AT&T profile page and AT&T has a great guide for this here. Doing this, you won't need to enter your ordinary email password to Spamdrain, but instead you will use a special code generated for Spamdrain only - the Secure Mail Key.
When you've created the Secure Mail Key, you use this as password when adding the email address to Spamdrain.
How do you know if your email address is AT&T based?
If your email address ends with any of the following domain names, you should log in to AT&T and create a Secure Mail Key as described above.