Exchange 2010 – Distribution Groups Not Accepting Internal Mails via SMTP

(I'm basing this on Exchange 2007, so apologies if it has changed significantly, I haven't looked at 2010).

• In Exchange System Manager, pull up Server Configuration -> Hub Transport -> Receive Connectors Tab.
• Check the Authentication tab. There's a few settings there that deal with the types of auth the server requires.

For testing, set up a new receive connector on a different port to experiment with the settings. Make the failing server use that, instead.

And + 1 to Evan for Wireshark. If you're not using SSL, you can watch the entire SMTP conversation between the web app and the receive connector. It's the best way of finding where the too are diverging in their behaviour.

A few ideas:

• You could set the groups as dynamic dist groups to prevent expansion, and set them up as moderated to prevent unauthorized mail.
  • Or just set a hard filter of allowed senders if that'll fly - but, that won't work for external users without disabling authentication.
• You can strip the reply-all button off of Outlook via group policy for internal users.

Nothing here stops users who know who's in the list from just sending mail to them.. but you can prevent casual exposure of that if it's non-trivial to expand the group to its members.