Microsoft Exchange

Restrict App Registration application permission to certain mailboxes

Requirements: Active Exchange Administrator Role and App Registration with application permissions granted.

Application Access Polices

Generally Application Permissions allow an Azure App Registration to access a certain type of data within the whole tenant.

For example the Application Permission Calendars.ReadWrite grants access to every calendar in every mailbox in the Exchange Online environment.

Use case

If, for example, you want to grant an App Registration the Read and Write permission on just 15 calendars, you can do so with an Application Access Policy.

Creating an Application Access Policy

To create an Application Access Policy, you first have to create your App Registration and grant the any Application permission (e.g. Mail.Send).

After that you need to create a Mail Enabled Security Group in the Exchange Admin Center and add the Mailboxes, on which the App Registration' permission shall be activated.

Now you can create the Application Access Policy with the following Command. First you have to log in to Exchange Online.

Import-Module ExchangeOnlineManagement
Connect-ExchangeOnline
New-ApplicationAccessPolicy -AppId "<appregistrationid>" -PolicyScopeGroupId "<PrimarySMTPAddressofMESG>" -Description "<yourcustomdescription>" -AccessRight Restrict

View existing Application Access Policies

If you want to view all existing Application Access Policies in your Tenant, you can do so with this command.

Get-ApplicationAccessPolicy

For more readability you can view this output in a gridview.

Get-ApplicationAccessPolicy | Out-GridView


Share Exchange Resources with external users

This guide describes how to share resources from your own environment with external persons (e.g. partners or office members). Internal persons can be invited or adapted in the same way. However, these instructions refer to collaboration outside the company.

Sign in to Exchange Online

The settings are only available via PowerShell and cannot be found in the Exchange Admin Center (WebGUI). Accordingly, you need the PowerShell module “ExchangeOnlineManagement”.

Install PowerShell module

Open a new PowerShell window with administrator rights and execute the installation command:

Install-Module -Name ExchangeOnlineManagement -Force

Connect to Exchange Online

You must then open a new PowerShell session. This should not have administrator rights and must be started from scratch (do not use an existing window, as the available module paths are not loaded correctly there).

Import-Module ExchangeOnlineManagement
Connect-ExchangeOnline

A browser pop-up will then appear. Follow the instructions and log in with your administrator account. The user should have Exchange Admin permissions.

This module contains many functions for the Sign In options. You can find more information here: Connect to Exchange Online PowerShell | Microsoft Learn

Get folder settings for calendar

This command displays all settings. Here you can check the current status of the configuration and your subsequent adjustments.

Get-MailboxCalendarFolder -Identity <youremailadress>:\calendar

Share calendar state publicly

This command configures the external sharing of calendar availability. The corresponding HTML and iCAL links are then displayed in the overview (execute the previous command again). Look for the parameters "PublishedCalendarUrl" & "PublishedICalUrl".

Set-MailboxCalendarFolder -Identity <youremailadress>:\calendar -PublishEnabled $true

Set shared details for calendar events

The availabilities are now displayed on the ICAL and HTML accesses by default. More details are not available. You can change the DetailLevel to these three settings so that external persons receive more or less information about the calendar.

Set-MailboxCalendarFolder -Identity  <youremailadress>:\calendar -DetailLevel LimitedDetails

Allow external calendar booking

With this command you can allow the booking of external mail addresses. If this is executed, all mail requests for resource accounts are answered and a confirmation mail is sent back if available. Take care to only enable this feature on calendars that are intended to be public.

Set-CalendarProcessing -Identity <youremailadress> -ProcessExternalMeetingMessages $true