Spamdrain normally works great with AT&T based email addresses, ending at att.net, ameritech.net, bellsouth.net, but in order to make the experience as safe and reliable as possible you'll need to 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.