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.