Had a customer and the newer employee was getting a error that they didn’t have permission to send email from a shared mailbox.  Here is how we were able to resolve this.

• Log into the Office 365 account with the Admin credentials
• Under “Groups” go to “Shared Mailboxes”
• Click on the mailbox to open it’s properties
• Under the “Members” section click “Customize Permissions”
• Under the “Send as” section click “Edit”
• Click “Add Permissions” and check the user you need to add and click “Save”.

Let us know if this helped you…