Script repository

$cancelReason = "You are allowed to modify only the primary SMTP address" # TODO: modify me

# Get Exchange properties set by the action.
$modifiedMailboxParams = $Context.Action.MailParameters
if (-not($modifiedMailboxParams.EmailAddressesModificationEnabled))
{
    return
}

# Get the modified e-mail addresses.
$modifiedAddressesCollection = $modifiedMailboxParams.EmailAddresses
if (-not($modifiedAddressesCollection.OverrideOldValues))
{
    $Context.Cancel($cancelReason)
    return
}

$modifiedEmailAddresses = New-Object "System.Collections.Generic.HashSet[System.String]"([System.StringComparer]::OrdinalIgnoreCase)
for ($i = 0; $i -lt $modifiedAddressesCollection.Count; $i++)
{
    $operation = "ADS_PROPERTY_NONE"
    $modifiedEmailAddress = $modifiedAddressesCollection.GetAddress($i,[ref]$operation)
    $modifiedEmailAddresses.Add($modifiedEmailAddress)
}

# Get the current e-mail addresses.
$mailboxParams = $Context.TargetObject.GetMailParameters()
$emailAddresses = $mailboxParams.EmailAddresses

# Compare the number of e-mail addresses.
if ($modifiedAddressesCollection.Count -ne $emailAddresses.Count)
{
    $Context.Cancel($cancelReason)
    return
}

# Compare the lists of the modified and current e-mail addresses.
for ($i = 0; $i -lt $emailAddresses.Count; $i++)
{
    $operation = "ADS_PROPERTY_NONE"
    $emailAddress = $emailAddresses.GetAddress($i,[ref]$operation)
    $modifiedEmailAddresses.Remove($emailAddress) | Out-Null
}

if ($modifiedEmailAddresses.Count -ne 0)
{
    $Context.Cancel($cancelReason)
    return
}