Apple Mail won't create new mail account

Tahoe -- Apple Mail cannot create account using wifi authentication failure

Over the past several months, my Apple devices, one-by-one,

have stopped working with my main email account over wifi. My iPhone 15 can

connect over cellular, and my iPad Mini 6 can connect when using my phone’s

hotspot. But my Intel-based iMac, the last to lose its ability to connect to

the mail server, is an utter failure. I should note that each device is running

the latest OS it can use (the iMac can’t go beyond Sequoia), and that each was

previously able to receive and send email in this account. None of these devices

has a problem using my Gmail accounts.

  I have tried all of the fixes I can find in these forums and elsewhere – removing VPN software,

removing and recreating the account in Apple Mail, disabling

security/anti-virus software and the firewall, etc.

  I just got a new iMac running Tahoe 26.1, and have delayed installing security/anti-virus

software in the hope that I could create the email account on a new machine

that has no software that might interfere. But I continue to be told that Apple

Mail cannot authenticate the account. I was able to save the account settings

  I don’t think any settings on my router are causing the problem, and they have not changed in a

long time – long before my devices began having problems. I have found no posts

indicating that these issues can be caused by the router.

  I have similarly found no posts identifying mail server settings that might be problematic. But if

that mnight be the cause I can change them. My mail server is running on a

system with Debian Bullsey with current versions of Postfix and Dovecot with

SSL, so it uses port 993 incoming and 587 outgoing.