we have a case here, where we have a project Team, based on internal and external User. Now we have to send emails from one Email address (O365 Group, Distribution List, Shared Mailbox) to our customers.
But i can not grant Send As permission to the external User for a Group.
I could create an internal User with a ExO Plan1 license, but in this case the external User are able to connect with this user to our tenant and might be able to see and join open Teams or can get access to Sharepoint sites which are allowed for all internal Users. I've tested this and even a user without any license can get access to non private teams.