To disable automapping, use Windows PowerShell to remove full access permissions from the user for the mailbox, and then add back full access permissions to the user. When you add back full access permissions to the user, use the AutoMapping:$falseparameter.

To do this, follow these steps:

1. Connect to Exchange Online by using remote PowerShell. For more info about how to do this, go to the following Microsoft website:
   Connect to Exchange Online Using Remote PowerShell
2. Remove full access permissions for the user from the mailbox. This removes automapping. To do this, at the command prompt, type the following command, and then press Enter:

   Remove-MailboxPermission -Identity <Mailbox ID1> -User <Mailbox ID2> -AccessRights FullAccess

   Note In this command, <Mailbox ID1> represents the mailbox to which the user is granted permissions, and <Mailbox ID 2> is the mailbox of the user from whom you want to remove full access permissions.

   For example, to remove full access permissions for an administrator from John Smith’s mailbox, use the following command:

   Remove-MailboxPermission -Identity [email protected] -User [email protected] -AccessRights FullAccess

   After you run this command, you’re prompted to confirm the action:

   Confirm
   Are you sure you want to perform this action?
   Removing mailbox permission “[email protected]” for user “[email protected]” with access rights ‘FullAccess'”.
   [Y] Yes [A] Yes to All [N] No [L] No to All [?] Help (default is “Y”): y
3. Grant full access permissions back to the user for the mailbox, but don’t enable automapping. To do this, at the command prompt, type the following command, and then press Enter:

   Add-MailboxPermission -Identity <Mailbox ID1> -User <Mailbox ID2>-AccessRights FullAccess -AutoMapping:$false

   Note In this command, <Mailbox ID 1> represents the mailbox to which the user is granted permission and <Mailbox ID 2> is the mailbox of the user to whom you want to add full access permissions.

   For example, to add full access permissions for an administrator to John Smith’s mailbox, type the following command, and then press Enter:

   Add-MailboxPermission -Identity [email protected] -User [email protected] -AccessRights FullAccess -AutoMapping:$false

   After you run this command, the following output is displayed.