In Microsoft 365/Teams, guests are external users who are identified by their external email addresses (e.g., guest@otherdomain.com). Creating an email address using your domain (e.g., guest@yourdomain.com) generally involves creating a user account in your Entra tenant. This makes them an internal member, not a guest.
As a workaround, you can create a contact with your domain-based email address (e.g. guest@yourdomain.com) that forwards to their external address (guest@otherdomain.com)
More at https://learn.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-mail-contacts
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hth
Marcin