It could be because our ISP is Spectrum -- it was Time Warner, then split off to become Brighthouse, then merged into Spectrum, all with the same email infrastructure from Roadrunner. Or it could be because today is Friday the 13th. Or maybe Windows Mail is broken in this version of Win11? Whatever the cause, on my mini-PC with the *possible* Win11 22H2 RTM, I could not add email accounts in the Mail app -- I tried for a few hours. Setting up my Gmail account did work, so I know the app itself was working enough to send & receive email.
The fix I found, which is undocumented anywhere AFAIK, was to export the following registry key from a working copy of Win10 with the email accounts set up in Windows Mail -- Note: the last number may be different with your copy of Windows:
[HKEY_CURRENT_USER\ SOFTWARE\ Microsoft\ ActiveSync\ Partners\ {3902330A-C09B-4CC1-9BF0-71AAAF54E4FE}]
On the mini-PC I double clicked the exported .reg file, adding its contents to the registry. Then I was able to add those email accounts normally in Windows Mail, & they worked (!).