You may receive any of the following errors attempting to open another mailbox that you have been granted permissions to:

• "this message could not be sent. You do not have the permission to send the message on behalf of the specified user."
• “you do not have permissions”
• “cannot display the Inbox folder”

There may be additional similar errors not listed here as well.

Newly added permissions to shared resources can take up to 48 hours to take effect for Microsoft 365 cloud-based mailboxes. One of the reasons for this delay is that these permissions rely on Microsoft and your email client to make regular automated updates to the Offline Address Book. You can attempt to speed up this process by forcing your email client to download a new copy of the Offline Address Book.