Dear Ken_072
Thank you for posting in the Microsoft Community.
I understand that you are unable to add your IMAP mailbox in the new Outlook.
I would like to ask if you used the application password to log in to your account during configuration? Because some email providers require that you do not use the email password when logging in to third-party email software such as Outlook, and you need to use a one-time application password to log in.
You can ask your email provider whether you need to turn on certain settings for logging in to third-party email software. For example, Gmail requires you to check the option to allow third-party emails to access Gmail content before you can log in. You can also ask your email provider's technical support for specific operations for email configuration. For example, the settings of incoming and outgoing servers and port numbers.
The new Outlook is still being improved and adapted to third-party mailboxes that are not in Microsoft domain names. It can be well adapted to mailboxes from companies such as Gmail or Yahoo. However, for some uncommon third-party IMAP mailboxes, the new Outlook may not be fully adapted yet. If you have the right to use the classic version of Outlook, you can try to use the classic version of Outlook to configure it. After years of development, the classic version of Outlook is more adaptable to third-party mailboxes. The new Outlook is also constantly improving and enriching its functions.
I hope that my reply will be helpful to you.
Please feel free to let me know if you have any further updates, thank you.
Best Regards,
Simon.W - MSFT | Microsoft Community Support Specialist