MacOS standard Apple email client, incoming mail arrives in trash folder. How to stop this?

Apologies if this is the wrong place to ask. (Can you suggest a better one?) I'm setting up a brand new Macbook Air (Oct 2022) for an older neighbor. The ISP is att.net The email protocol is standard SMTP, and the server is smtp.mail.att.net . One quirk: AT&T uses OAuth to verify client connections, and they have a page telling you to set up a "secure key" to access your email. The webpage describing that is [https://www.att.com/support/article/email-support/KM1240308/](https://www.att.com/support/article/email-support/KM1240308/) . I did that for the old Macbook that this new one is going to replace. And carried it forward to the new Macbook, which does support OAuth in the current email client, but I left the OAuth avoidance procedure in place. The problem (on both old and new Macbooks) is that incoming email arrives in the Trash folder instead of in the Inbox folder. I was on the phone with AT&T support in the Phillipines for an hour before I finally realized that they knew nothing whatever about email on any level, but would be nice to me for as long as it took for that to sink in. We've been moving all the incoming email into the Inbox manually and everything works perfectly from there. Any ideas on how to fix this? I set up a test account (with neighbor's credentials) on a different Apple computer, and the new test email account has exactly the same problem. Next I will try removing the OAuth avoidance protocol and reverting to the older process. Web searches reveal that other people have hit this issue, but no good remedy has been found so far. Help!