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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 has the following permissions scope . 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 -SendingAddress -To -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 -SendingAddress -To -Subject "This is a Test Message" -Body "Test Body" -AttachmentFileName "c:\temp\olm.csv" -userName -ClientId {your AzureApp Registration Id} -RedirectURI {Your redirect URI}

Which will produce a conversation like


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 module that looks pretty good, with C# here is a simple example that use MSAL and MailKit

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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.