1. Use Get-MailboxPermission to verify the permission level
Get-MailboxPermission -Identity "Mailbox Name" -User username | fl
2. Next, use Remove-MailboxPermission to remove the mailbox permission for the user.
Remove-MailboxPermission -Identity "Mailbox Name" -User username -AccessRights FullAccess
3. Re-add the mailbox permission with the -AutoMapping switch
Add-MailboxPermission -Identity "Mailbox Name" -User username -AccessRights FullAccess -AutoMapping:$false
4. Allow some time for user to pickup that via Outlook Autodicovery
5. To view the list of auto-mapped users for a mailbox run:
Get-ADUser -Filter {Name -eq "Mailbox Name"} -Properties msExchDelegateListLink | Select -ExpandProperty msExchDelegateListLink
Source: here
