Mail relay to Office 365 from IIS SMTP Relay

Occasional Contributor

Since about a month we cannot relay email from our IIS SMTP Relay to Office 365. This has worked for a long time, therefore I do not get why it is not working anymore. The account used is still active and Iám able to sign-in with the account. I've even completely reinstalled and reconfigured the SMTP relay role in IIS following this procedure:



I'm getting the following warning in the eventvwr:

Message delivery to the host '' failed while delivering to the remote domain '##########.COM' for the following reason: The remote SMTP service rejected AUTH negotiation.


Hope you guys can help me out here.

10 Replies

Hello, Ronald,


You might want to checkout the article below, it has some additional troubleshooting and configuration guidance.


Setting up multifunction device or application to send email using Office 365


Let me know if it helps. 



How is the handshake protocol being implemented?  Kerb?  CHAP?, etc....FS?



We have the same message.

Some mails wont get thru i thougt it was problem on the other side.


last week we had some problems .

we have a hybrid setup i think is setup right.

how come some mails work and others not ?


Note that this will be forced only from 1st February, so the root cause is definitely different here.

@Daniel Lentz wrote:

Hi, there have been made som changes starting 2017

Be sure to read the scenarios very carefully as it doesn't apply to all IIS SMTP Relay setups.

Verify you are not using "Integrated Windows Authentication" and you are using "Basic Authentication" for Outbound Security.  Also the "Domain Name" should not match your Email Domain since you want to email to send via the Smart Host.

@Ronald van Ackooij 


Hi, this is an old post but I got this same issue. I had to enable the SMTP Auth option at the mailbox level, on Office 365:


Enable SMTP AUTH for specific mailboxes
  1. Open the Microsoft 365 admin center and go to Users > Active users.
  2. Select the user, and in the flyout that appears, click Mail.
  3. In the Email apps section, click Manage email apps.
  4. Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.
Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.

Step 4 must be enabled. in order to work ?
works for me, ty