Hi Dylan21,
Did you mean that you have changed your email address to username@yourdomain.com, but cannot add this account in Outlook client?
Have you added DNS records for Office 365 in your custom domain?
For this scenario, Please try to run the Domain Troubleshoot Wizard:
Choose your second domain from Domain tab in Office 365 MOP page, click troubleshoot, and choose "I changed the settings at my provider more than 72 hours ago".
Additionally, access Outlook Web App (OWA) and check whether you can receive emails with your account via OWA.
Thanks,
Bourne Zhang