Remove automapping for a shared mailbox in Office 365

In short, there is no simple way to remove automapping from a shared mailbox in Office 365 unfortunately. But there is a more tedious way 😉

First remove the user from the mailbox, then re-add the user without automapping enabled.

Fire up PowerShell and connect to your Office 365 tenant with:

Connect-ExchangeOnline -UserPrincipalName your.name@yourcompany.com -ShowProgress $true

Remove the user from the shared mailbox delegation:

Remove-MailboxPermission -Identity shared.mail@example.com -User user.mail@example.com -AccessRights FullAccess

Then add the user to the shared mailbox delegation again, without automapping:

Add-MailboxPermission -Identity shared.mail@example.com -User user.mail@example.com -AccessRight FullAccess -InheritanceType All -Automapping $false

If you need to give the user send as permission too you can do:

Add-RecipientPermission shared.mail@example.com -AccessRights SendAs -Trustee user.mail@example.com

Sources:

https://support.microsoft.com/en-us/help/2646504/how-to-remove-automapping-for-a-shared-mailbox-in-office-365