When collaborating with external partners, you most likely created B2B guest accounts in your tenant. You actively invited them using Azure Active Directory or added them to your Teams in Microsoft Teams. 
When your partner companies change their email domains, e.g., changing the primary domain brand, you might end up in a situation where you cannot send email messages any more. They receive a non-delivery notification, even when using the new email address.
During this session, you learn about the reasons for this by-design feature of Entra ID and how to resolve it in your tenant.