I am getting an error when trying to end an email using outlook through another users email. I have set up permission in the microsoft admin for my email to send through but I get the same error. For context, I do this with another branch of our organization and set up permission for that one and never have an issue. With this office though, I have never been able to get it to work.
Hi
Welcome to the community. I apologize if you are running with some errors. Upon checking with the error, it may be because of the shared email causing this error. Please check the “From Email” address field. The connected Outlook account must have the necessary permissions to send the email as the (shared) email address entered into “From Email”
Let us know if that fixes your issue. We’ll be looking forward for your response. Thanks.
We figured it out! Turns out you need full mailbox access in the admin console to make it work.
Great job,
If you have any more questions, feel free to reach out to the Community. We're always here to help! 
So, in the admin console. Go to Users → Active Users → Select the user you want access to → Go to the Mail tab on the user → Then select “Read and manager permissions” under Mailbox permissions.
That should do the trick!
Enter your E-mail address. We'll send you an e-mail with instructions to reset your password.