Office 365 and IMAP or POP3 with OAuth 2.0 authentication in unattended (app-only) mode: How to make it work

  |   Lukas Matyska

On October 1st 2022, Microsoft will begin disabling Basic authentication in Microsoft 365 for IMAP, POP3 and EWS. This means that classic username/password authentication will no longer work with Exchange Online, and application will have to be upgraded to use OAuth 2.0.

Microsoft 365 (formerly Office 365) supports two kinds of OAuth 2.0 authentication:

  • Delegated authentication is suitable for desktop, mobile or web applications with signed-in user present.
    This mode is described in detail in another article.

  • App-only authentication is suitable for services or daemons with no user present. Instead, these unattended applications authenticate using client secrets (application credentials) to receive an access token, which is then used to gain access to a mailbox using IMAP, POP3 or EWS protocols.

Before your application can start accessing mailboxes, you have to register it with Microsoft, assign the relevant permissions and configure mailbox access. The guide below describes what needs to be done to enable POP3 or IMAP access for unattended apps (app-only mode). For EWS, see Office 365 and EWS with OAuth 2.0 in unattended mode.

Register yourself and your company

  1. Log into Azure Portal. If you don't have an account there yet, create it. You also have to set up a tenant that represents your company.

  2. If you administer more than one tenant, use Directories + subscriptions filter to select the tenant for whom to register an application.

    Create new app registration

Register your application

  1. In Azure Portal ⇒ expand the left menu ⇒ select Azure Active Directory ⇒ select App registrations ⇒ click + New registration. (Azure Portal is constantly evolving, so if you cannot find this page, use the search bar.)

    Creating app registration 1
  2. Name your application, choose which kind of accounts are going to use it, and click Register.

    Note: This guide is suitable for single tenant account types. For other types, further steps might be different.

    Creating app registration 2
  3. You successfully registered your application and you can view its associated IDs. Some of them will be needed later to obtain an OAuth 2.0 token.

    Creating app registration 3

Set up client secret (application password)

  1. In the left menu, select Certificates & secrets ⇒ click + New client secret.

    Adding client secret 1
  2. Provide some description for this secret, choose expiration period, and click Add.

    Adding client secret 2
  3. Immediately copy and save the newly created client secret's Value (not Secret ID). You will not be able to view the Value later anymore.

    Adding client secret 3

Add app permissions

  1. In the left menu, select API permissions ⇒ click + Add a permission.

    Adding app permissions 1
  2. Navigate to APIs my organization uses tab ⇒ type Office 365 Exchange in the search bar ⇒ click Office 365 Exchange Online entry.

    Adding app permissions 2
  3. Click Application permissions ⇒ type AccessAsApp ⇒ check IMAP.AccessAsApp and/or POP.AccessAsApp ⇒ click Add permissions.

    Adding app permissions 3
  4. The newly-added IMAP.AccessAsApp and POP.AccessAsApp permissions have to be approved by your organization's administrator. Ask them to grant consent to your application by clicking Grant admin consent for [organization].

    Adding app permissions 4
  5. Application permissions have been granted. Optionally, you can remove the delegated User.Read permission which is not needed for app-only application - click the context menu on the right side of the permission and select Remove permission.

    Adding app permissions 5

Add mailbox access permissions

  1. Now, you have to assign access permissions for your mailboxes. There is no web UI for this yet - you have to use PowerShell.

  2. Install the required PowerShell modules.

    You can skip this step if you have already installed AzureAD and ExchangeOnlineManagement modules.

    Open your PowerShell as Administrator, and run:

    Install-Module -Name AzureAD
    Install-Module -Name ExchangeOnlineManagement

    Confirm installation from PSGallery by typing Y + Enter.

    Adding mailbox permissions 1

    (Wondering why these modules install from an untrusted repository? See this answer to Azure-PowerShell issue.)

  3. Get the service principal ID associated with your application.

    Note: You will be asked to log into your Azure account.

    $AppId = "YOUR_APP_ID_HERE"
    $TenantId = "YOUR_TENANT_ID_HERE"
    Import-module AzureAD
    Connect-AzureAd -Tenant $TenantId
    ($Principal = Get-AzureADServicePrincipal -filter "AppId eq '$AppId'")
    $PrincipalId = $Principal.ObjectId
    Adding mailbox permissions 2
  4. Register the service principal for your application.

    Note: You will be asked to log into your Exchange Online account.

    $DisplayName = "Some principal name for IMAP/POP3 here"
    Import-module ExchangeOnlineManagement
    Connect-ExchangeOnline -Organization $TenantId
    New-ServicePrincipal -AppId $AppId -ServiceId $PrincipalId -DisplayName $DisplayName
    Adding mailbox permissions 3
  5. Add FullAccess mailbox permissions to all mailboxes you want to access from your application.

    Add-MailboxPermission -User $PrincipalId -AccessRights FullAccess -Identity ""
    Add-MailboxPermission -User $PrincipalId -AccessRights FullAccess -Identity ""
    Add-MailboxPermission -User $PrincipalId -AccessRights FullAccess -Identity ""	
    Adding mailbox permissions 4
  6. Congratulations! Now you have registered an application for accessing Office 365 mailboxes via IMAP or POP3 protocol and received its Application (client) ID, Client secret and Directory (tenant) ID.

    These strings are going to be used by your application to authenticate to Microsoft 365 via OAuth 2.0 and receive an OAuth token. This token is then used to authenticate to Exchange Online using IMAP or POP3 protocols.

Let's write some code!

  1. In your application, add reference to Microsoft.Identity.Client package, and specify your IDs, the secret, and the e-mail address. Specify the scope to request a token as well (use for app-only mode).

    // application (client) ID obtained from Azure
    const string ClientId = "4f5a9f88-1111-1111-1111-bdca9a88c089"; // change to your AppId
    // application's 'client secret' value (application password)
    const string ClientSecretValue = "ThisIsSomeVerySecretValue"; // change to your secret value
    // your organization's directory (tenant) ID
    const string TenantId = "fb561382-2222-2222-2222-06ab992d36b7"; // change to your TenantId
    // mailbox to access (not an alias)
    const string SmtpAddress = ""; // change this
    // default scope of permissions to request
    static readonly string[] Scopes = new[] {
        "", // for accessing Exchange Online with app-only auth
  2. Before your application connects to Office 365's IMAP or POP3 service, it has to request an OAuth 2.0 access token using the Microsoft's authentication API.

    using Microsoft.Identity.Client;
    // get an instance of 'confidential client application' API
    var cca = ConfidentialClientApplicationBuilder
    // acquire OAuth 2.0 access token from Microsoft 365
    AuthenticationResult result = await cca.AcquireTokenForClient(Scopes).ExecuteAsync();
    string accessToken = result.AccessToken;
  3. Then, connect to using Rebex IMAP or Rebex POP3 package and use the acquired token to start accessing your organization's mailboxes.

    using Rebex.Net;
    // connect to Office 365 and authenticate using access token
    var client = new Imap(); // or new Pop3()
    client.Connect("", SslMode.Implicit);
    client.Login(SmtpAddress, accessToken, ImapAuthentication.OAuth20)
    // start working with the mailbox

To give this a try before adding the code to your application, try our ImapOAuthAppOnlyConsole or Pop3OAuthAppOnlyConsole samples.

Documentation for IMAP and POP3 .NET libraries

Any issues?

  • Make sure you used correct IDs in PowerShell cmdlets. If the IDs are mismatched, your app will be able to request an access token, but won't be able to use it to access mailboxes.

  • Use an up-to-date version of Rebex IMAP or POP3. Old versions have not been tested with contemporary Exchange Online. They might still work, but if you encounter any issues, please try the latest release.

  • This guide is only suitable for IMAP and POP3. For Exchange Web Services guide, see Office 365 and EWS with OAuth 2.0.

  • Microsoft 365 does not support app-only authentication for SMTP yet. However, it will still be possible to enable username/password authentication for SMTP after October 1st 2022.

  • Need help? Ask at Rebex Q&A Forum.