Att.net imap problem on Kindle Fire

I am having an issue with my email on my Kindle Fire. I have an @att.net email account and have tried using multiple email apps to access the email via IMAP using the following: imap.mail.att.net and smtp.mail.att.net

I keep getting either a "incorrect username/password" or "Server not responding" errors when trying to connect to this account. I know the username and password work fine as I can access the account via webmail.

To troubleshoot this issue, I have updated my Kindle OS and deleted/attempted to reinstall the account on the device and still no luck. Is something wrong with the IMAP server? What can I do to fix this issue?