Get-SharedMailboxAutoMappingReport

<#
.SYNOPSIS
    Reports automapping status for delegates on shared mailboxes.

.DESCRIPTION
    For every shared mailbox, lists Full Access delegates and whether
    automapping was enabled when that permission was granted, since Outlook
    only auto adds a shared mailbox when AutoMapping was true at grant time.

.EXAMPLE
    .\Get-SharedMailboxAutoMappingReport.ps1

.NOTES
    Requires ExchangeOnlineManagement and an active Connect-ExchangeOnline session.

.AUTHOR
    Shehryar Hassan
#>

$sharedMailboxes = Get-Mailbox -ResultSize Unlimited -RecipientTypeDetails SharedMailbox
$report = foreach ($mbx in $sharedMailboxes) {
    $perms = Get-MailboxPermission -Identity $mbx.Identity |
        Where-Object { $_.AccessRights -contains "FullAccess" -and $_.User -notlike "NT AUTHORITY\*" }
    foreach ($perm in $perms) {
        [pscustomobject]@{
            SharedMailbox = $mbx.DisplayName
            Delegate      = $perm.User
            AutoMapped    = $perm.IsInherited -eq $false -and $perm.AutoMapping
        }
    }
}

$report | Sort-Object SharedMailbox | Format-Table -AutoSize