Recently, I had the unpleasant experience of getting "Your username or password is not accepted" when attempting to access my Gmail account with Windows Live Mail. However, I had the correct username and password and none of the settings in my email client had changed. This was a very perplexing situation indeed. Previously, I had no problems retrieving or sending email with this account for over a year.
I should have realized immediately that Google had something to do with the problem. Google changed their policy of allowing less secure apps to access Gmail on 5/30/2022. The problem was the "access permissions" needed for Windows Live Mail. Despite the fact that Windows 10 comes with "Windows Mail" built-in, I had continued using "Windows Live Mail" as the computer was upgraded from Windows 7 and I prefer Windows Live Mail as a mail client. Live Mail is considered a "less secure" app and Google wants to place the responsibility for any security problem or loss of data squarely on the user.
In my case, I wasn't sure whether a setting for the email account had been changed accidentally. Unfortunately I had forgotten to screen capture the settings for that particular email account. Therefore, I spent hours trying various settings, changing the account back and forth between POP and IMAP. Of course, the settings for the account had absolutely nothing to do with the problem, so I kept getting the same error no matter what was changed. I had also tried turning off the Windows Firewall and turning it back on, but no go.
This YouTube video shows how to re-enable access for a "less secure" app to access a Gmail account:
https://www.youtube.com/watch?v=2IyeDqXNal8
I actually re-enabled access for Windows Live Mail by allowing Windows Mail to access the email account. However, the steps shown in the video are the proper way to go about allowing a less secure app to access Gmail because the procedure involves setting up double-factor authentication.