Unable to verify account name or password

After changing potentially compromised password, the Apply Mail email account would not Verify.

This issue has been asked several times both here and Reddit. The solutions offered include:

• deleting the email account and create a replacement account. Disadvantage, you will need to download all old emails again from the server (including those junk emails deleted historically). As I didn't want to download again GB of old emails, this was not an option for me.
• this solution (didn't work for me): Mail is "Unable to verify account name or… - Apple Community

I checked that it was possible to log into the email account using webmail i.e. the new password did work. I also changed the password in my email app on the iphone and was able to download a test email sent to the problematic account.

These steps proved the problem was restricted to my Apple MacBook Air.

Looking in the KeyChain Access, I could see that this still retained the old email account password for both smtp and imap.

To check this yourself:

1. Spotlight Search : Keychain Access.app
2. Select "Open Keychain Access" in the popup dialog
3. In the menu on the left, choose System Keywords/System
4. Enter smtp in the search bar.
5. Double click each of the "com.apple.account.SMTP.password" entries to look for the email account with the verify problem.
6. When this is found, click Show Password. If this is the old password, replace this with the new password and save.
7. Repeat this for the imap settings i.e. Start at 4. and replace smtp with imap.

Once you have changed the smtp and imap password, you will need to reboot to reload the keychain access file.

This process solved the problem for me. Hope that it helps others.