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Check this updated ATT mail setup on Windows Mail app guide with detailed instructions and you will be able to configure your email on all mail application(outlook, thunderbird etc) and devices (phone, tablets etc).

Check ATT Email Using IMAP & SMTP Settings Manually

Open Outlook or Thunderbird on your device.
Go to account settings and choose “Add Account” option.
Then choose “Others” from the list of email providers.
After that, type your right email address in username section followed by your email “password” in the password field.
Now go to the “Manual Setup” option and choose on manual settings and enter them as shown below-

IMAP server: “imap.mail.att.net”

Security type: None

Imap port: 993

Secure: Port=993 and Security type: SSL/TLS (Accept all certificates)

Insecure: Port=143 and security type: None (Accept all certificates)

Now type the mentioned Outgoing (SMTP) settings and thereafter choose "Next".

Outgoing (SMTP) secure=Port 465

Security Type: SSL/TLS(Accept all certificates)

Outgoing (SMTP roaming server) Secure=Port 587 and security type: SSL/TLS(Accept all certificates)

Outgoing (SMTP)= Port 25 and Security type= None(accept all certificates)

Outgoing (SMTP roaming server) Insecure= Port 587 and Security type= None (accept all certificates)

Now to finish off the complete procedure, choose “Done” and you are good to use your ATT email account.

Read more - Setup ATT mail on Windows Mail app