I used to have email via ATT following several mergers. The problem is that mail is frequently hacked and then used for SPAM and malicious reasons, so they've been making their servers more secure. A lot of mail apps did not support the required security. If you get an app that does support the required security, you shouldn't need secure keys.
https://www.att.com/support/article/email-support/KM1240462
You need an app that supports OAuth. I don't know if there are any other than what's in the green box at the top.
What you probably DON'T want (and they'd probably prefer that you NOT use) are apps in the Red box. They will require the Secure key.
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Steve
'08 Boxster RS60 Spyder #0099/1960
- never named a car before, but this is Charlotte.
'88 targa
SOLD 2004 - gone but not forgotten