Adding and Removing Exchange Mailbox Delegate Access
It's a pretty common request to add or remove delegate access to an Exchange mailbox. However, if you do it through the Exchange dashboard UI, it will add that delegated mailbox to the user's Outlook mailboxes. This becomes a problem if the user has delegate access to too many mailboxes, as Outlook will start to slow down as it processes the incoming and outgoing mail for all of those mailboxes.
Instead, you can use Powershell to add or remove delegate access to a mailbox without adding it to Outlook. By doing it this way, automapping can be disabled. The drawback is that the delegate mailbox will have to be opened via the OWA web interface, but this is a small price to pay for a faster Outlook experience.
You can direct your user to visit https://mail.office365.com and sign in with their own account. Once logged in, they can click their portrait at the top right, then select to open another mailbox. They can then open the mailbox they have delegate access to, and it will open in a new tab.