Outgoing emails are not working in SPS2016 after Security Update May 2017

This article has inform you previously there may be some concequences after May 2017 Security Update for SharePoint in some special configurations.

There is a security update May 9,2017 for SharePoint Server 2016
You can find details in following KB
https://support.microsoft.com/en-us/help/3191880/description-of-the-security-update-for-sharepoint-server-2016-may-9-20

Well it is confusing, as you may know, out of the box mail configuration for SharePoint always anonymous. Thats correct.
But in some special configuration applied by customers to force SharePoint processes (w3wp or owstimer) to authenticate with their identities to Exchange server;  If aspnet:AllowAnonymousImpersonation settings was disabled for Authenticated users (well it doesn’t work for anonymous users at all) it may work.

<appSettings>
<add key=”aspnet:AllowAnonymousImpersonation” value=”false” />
</appSettings>

More details explained for this.
https://support.microsoft.com/en-us/help/2686411/sharepoint-impersonates-the-iusr-account-and-is-denied-access-to-resources
Security Warning : Well the suggested action for this settings , this should be enabled. Otherwise anonymous request will have higher rights with Application Pool Identities does.

The problem of this kind of authentication is incorrect ,not expected  for SharePoint and Microsoft considered this is a Security Issue. As Microsoft said by design it has to be anonymous. With that Security fix will prevent this. SharePoint will be always use anonymous authentication through SMTP servers.

For customers who interested force authentication , well there’s no way to disable the anonymous-only behavior but we have valid workaround for that:

1. If you are using Exchange, you can set up a separate receive connector configured as externally secured, and restricted to the IP addresses of the SharePoint server(s) in their environment.  This will allow SharePoint to send e-mails anonymously through this receive connector, but the connector will treat the e-mails as if you were authenticated.  All other SMTP clients will continue using the regular receive connectors and any authentication policies associated with those receive connectors.
2. Set up a smarthost SMTP relay that will accept e-mails anonymously from the SharePoint server(s), and then relay them to the company’s SMTP infrastructure using authentication.