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TL;DR: How do I make sure that the SMTP server doesn't accept external messages for distribution groups that are set to require authentication?

I have an Exchange 2010 server that has multiple distribution groups where some of the groups are used only internally (RequireSenderAuthenticationEnabled == $true) and some are used publicly (RequireSenderAuthenticationEnabled == $false).

The public groups work as expected.

The internal groups work as expected for internal (i.e. authenticated) users, however, the SMTP service is not rejecting messages from external users. Instead, it accepts the message into the queue:

250 2.6.0 <4B9ED980-5B72-48A9-B393-2547E11B4EF8@senderdomain.com> [InternalId=3951] Queued mail for delivery

But then fails to route the message:

Source                  : ROUTING
EventId                 : FAIL
InternalMessageId       : 3951
MessageId               : <4B9ED980-5B72-48A9-B393-2547E11B4EF8@senderdomain.com>
Recipients              : {testgroup@recipientdomain.com}
RecipientStatus         : {550 5.7.1 RESOLVER.RST.AuthRequired; authentication required}

So the end result is that the message is never received by the group, however, the sender doesn't know this b/c according to our SMTP server the message was accepted.

How do I ensure that the SMTP server rejects the message instead of queuing it for delivery? (I thought this was the default behavior but I'm not sure what might have changed)

The relevant receive connector is set up as follows:

Authentication tab

Permission tab

Update 2019-03-19:

I have confirmed that running the same scenario on another environment (Exchange 2010 on SBS 2011) produces the expected results. I created two groups, set the RequireSenderAuthenticationEnabled flag accordingly, and the private group rejects external email at the SMTP connector with the following message:

550 5.1.1 User Unknown

The relevant SMTP connector has the same settings on the Authentication and Permission pages.

Update 2019-03-21:

After installing Exchange 2010 SP3 R26 the results are still the same.

  • What does your mail transport flow look like? Are external MXs sending to a mail filter, to an edge transport server, or to a hub transport server? – Ruscal Mar 22 at 16:03
  • External parties send directly to the Exchange server but traffic is filtered through an SMTP proxy on the firewall. Bypassing the SMTP filter by connecting directly to the Exchange server's SMTP port has the same results. – Jens Ehrich Mar 22 at 17:18
  • My memory is hazy on 2010 but I'm pretty sure that at least newer Exchanges (by default) accept the message and sent back a bounce at Routing step (like your error message). – Don Zoomik Mar 24 at 22:26

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