By default, Exchange Online in Office 365 will automatically map delegated mailbox's in Outlook. Depending on your requirements this may be the desired outcome, however, if it's not you'll notice that you can't prevent auto-mapping from the web interface. Thankfully Microsoft offer almost complete administrative control of your Office 365 Exchange Online instance via PowerShell. This can be helpful when users call in sick and you want to quickly provide someone in the organization access to their mailbox without initiating a sync of the delegated mailbox to their Outlook profile. This also helps to keep users Outlook profiles clean and local search running smoothly, avoiding uncessary bloat.
This tutorial assumes you know how to connect to your Office 365's Exchange Online Tenant with PowerShell. If you're unsure of how to do this please follow the steps here to get setup.
To delegate full access to a mailbox without automapping in Outlook:
Add-MailboxPermission -Identity <MailboxID> -User <User> -AccessRights FullAccess -AutoMapping:$false
<MailboxID> - The mailbox to which you want to provide access.
<User> - The user who needs access to the mailbox.
Automapping - $true or $false depending on whether you want the mailbox to appear in users local Outlook profile. $true will show the mailbox, $false will hide the mailbox.
The delegate can now access the mailbox via the below URL:
[email protected] - Is the email address