A new preview feature allows the resources available to an Azure AD guest account to be reassigned to another email address. It’s a nice feature, but Teams has some problems with it at present. On the upside, everything works great with SharePoint Online and Planner, and we’re sure that Microsoft will fix the problem with Teams soon.
https://office365itpros.com/2021/03/22/reset-email-account-azure-ad-guest/
