If you can't configure your custom email domain account as an Exchange account in Outlook, this workaround might help.
Workaround for the issue
Add a CNAME record to your DNS settings at your domain registrar. A CNAME record is needed for Outlook to configure the account using Autodiscover. The only record you need to add is for Host Autodiscover.
Verify that your MX record is still configured for Hotmail. An MX record ensures you can receive email from other people.
When you originally set up your domain settings for Outlook.com, an MX record was added to your DNS settings. This record has some unique numbers or letters followed by *.pamx1.hotmail.com, and it needs to be preserved. You can confirm the MX record is still configured properly by looking up your domain at https://mxtoolbox.com. This record should be listed before any other MX records for this domain. For example, this is what it should look like on https://mxtoolbox.com: