Skip to main content

Testing and Sending email via SMTP using Opportunistic TLS and oAuth in Office365 with PowerShell

As well as EWS and Remote PowerShell (RPS) other mail protocols POP3, IMAP and SMTP have had OAuth authentication enabled in Exchange Online (Official announcement here). A while ago I created this script that used Opportunistic TLS to perform a Telnet style test against a SMTP server using SMTP AUTH. Now that oAuth authentication has been enabled in office365 I've updated this script to be able to use oAuth instead of SMTP Auth to test against Office365. I've also included a function to actually send a Message.

Token Acquisition 

To Send a Mail using oAuth you first need to get an Access token from Azure AD there are plenty of ways of doing this in PowerShell. You could use a library like MSAL or ADAL (just google your favoured method) or use a library less approach which I've included with this script . Whatever way you do this you need to make sure that your application registration https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app has the following permissions scope https://outlook.office.com/SMTP.Send . One thing to note is that Application permissions aren't supported at the moment so you need to use one of the Delegate Authentication flows (which means having a service account for SendAs other user scenarios).


Adding the SASL XOAUTH2 header

This was really the only thing I needed to change in the initial script apart from adding in code to get the OAuth token. The SASL header looks like the following

base64("user=" + userName + "^Aauth=Bearer " + accessToken + "^A^A")

The ^A character is a Control code which relates to character 01 in the ASCII Character set which corresponds to SOH (Start of Heading).

Testing out OAuth


To test out oAuth against Office365 use something like the following

Invoke-TestSMTPTLSwithOauth -ServerName smtp.office365.com -SendingAddress gscales@datarumble.com -To gscales@datarumble.com -ClientId {your AzureApp Registration Id} -RedirectURI {Your redirect URI}

which should give you an output like
 

Actually Sending a Message

As well as the SMTP Mail Conversation Test function, I also included a function that would allow you to actually send an Email Message using SMTP,TLS and oAuth. As the System.Net.Mail.Message class is now obsolete (which also takes Send-MailMessage along with it in term of sending via oAuth) there is no way of easily sending a Message without a third party library like MailKit (which is actually a really good library and supports things like Dkim etc). To get the Message Send to work I used the System.Net.Mail.Message and then used reflection to substitute a MemoryStream into the Send function so I could get the Message from this class as a MimeStream. This stream can then be sent as part of the SMTP DATA verb  (minus the X-Sender/X-Reciever Headers).  As mentioned in the Token Acquisition if you want to send as another user you need to have the normal Exchange SendAS permission granted to the Delegate account you using. To Send a Message with an Attachment use something like

Invoke-SendMessagewithOAuth -ServerName smtp.office365.com -SendingAddress jcool@somedomain.com -To gscales@somedomain.com -Subject "This is a Test Message" -Body "Test Body" -AttachmentFileName "c:\temp\olm.csv" -userName gscales@datarumble.com -ClientId {your AzureApp Registration Id} -RedirectURI {Your redirect URI}

Which will produce a conversation like



MailKit

If your reading this post because you have existing code that needs to be converted to use oAuth then the library you want to use in either PowerShell or C# is MailKit . For Powershell I would check out the https://www.powershellgallery.com/packages/Mailozaurr/0.0.9 module that looks pretty good, with C# here is a simple example that use MSAL and MailKit


Labels:

Popular posts from this blog

The MailboxConcurrency limit and using Batching in the Microsoft Graph API

If your getting an error such as Application is over its MailboxConcurrency limit while using the Microsoft Graph API this post may help you understand why. Background   The Mailbox  concurrency limit when your using the Graph API is 4 as per https://docs.microsoft.com/en-us/graph/throttling#outlook-service-limits . This is evaluated for each app ID and mailbox combination so this means you can have different apps running under the same credentials and the poor behavior of one won't cause the other to be throttled. If you compared that to EWS you could have up to 27 concurrent connections but they are shared across all apps on a first come first served basis. Batching Batching in the Graph API is a way of combining multiple requests into a single HTTP request. Batching in the Exchange Mail API's EWS and MAPI has been around for a long time and its common, for email Apps to process large numbers of smaller items for a variety of reasons.  Batching in the Gr...
Read more

Sending a Message in Exchange Online via REST from an Arduino MKR1000

This is part 2 of my MKR1000 article, in this previous post  I looked at sending a Message via EWS using Basic Authentication.  In this Post I'll look at using the new Outlook REST API  which requires using OAuth authentication to get an Access Token. The prerequisites for this sketch are the same as in the other post with the addition of the ArduinoJson library  https://github.com/bblanchon/ArduinoJson  which is used to parse the Authentication Results to extract the Access Token. Also the SSL certificates for the login.windows.net  and outlook.office365.com need to be uploaded to the devices using the wifi101 Firmware updater. To use Token Authentication you need to register an Application in Azure https://msdn.microsoft.com/en-us/office/office365/howto/add-common-consent-manually  with the Mail.Send permission. The application should be a Native Client app that use the Out of Band Callback urn:ietf:wg:oauth:2.0:oob. You ...
Read more

How to test SMTP using Opportunistic TLS with Powershell and grab the public certificate a SMTP server is using

Most email services these day employ Opportunistic TLS when trying to send Messages which means that wherever possible the Messages will be encrypted rather then the plain text legacy of SMTP.  This method was defined in RFC 3207 "SMTP Service Extension for Secure SMTP over Transport Layer Security" and  there's a quite a good explanation of Opportunistic TLS on Wikipedia  https://en.wikipedia.org/wiki/Opportunistic_TLS .  This is used for both Server to Server (eg MTA to MTA) and Client to server (Eg a Message client like Outlook which acts as a MSA) the later being generally Authenticated. Basically it allows you to have a normal plain text SMTP conversation that is then upgraded to TLS using the STARTTLS verb. Not all servers will support this verb so if its not supported then a message is just sent as Plain text. TLS relies on PKI certificates and the administrative issue s that come around certificate management like expired certificates which is why ...
Read more
All sample scripts and source code is provided by for illustrative purposes only. All examples are untested in different environments and therefore, I cannot guarantee or imply reliability, serviceability, or function of these programs.

All code contained herein is provided to you "AS IS" without any warranties of any kind. The implied warranties of non-infringement, merchantability and fitness for a particular purpose are expressly disclaimed.