Black Hole Distribution Groups

I had the question posed to me today: if I have a mail-enabled group (either a security group or a distribution group) that has no members, and it receives an email, why don’t I get an NDR?

Well – works as intended!

Let’s have a thought experiment considering how mail-enabled groups work (from an Exchange perspective):

  1. Create a distribution group
  2. Assign it one or more email addresses
  3. Don’t add any members
  4. Send an email
  5. Let Transport expand the group
  6. For every member in the group, make a copy of the email
  7. Stop

What’s different about the empty group? Absolutely nothing. A copy of the email is made for every member of the group – it just so happens that there aren’t any members of the group!

Why would you do this? It’s commonly done for employees that have left a company. For some period of time, you may want an ex-employees supervisor or manager to receive their emails. But eventually, you just want them to “go away”.

It’s also commonly done for “temporary” email addresses. For example, your company decides to run a promotion, and your have your customers send an email to a custom temporary email address. When the promotion is done, black-hole the email.

Now, starting in Exchange 2007 you can also create custom responses with transport rules, if you need or want them.

Black hole distribution groups have worked this way since at least Exchange 5.5. A further note: it’s also common to hide these type of mail-enabled groups from the Exchange address book, so that they are not present in the GAL. Also be aware that you can have dozens of addresses assigned to a single group; so in most instances you’ll only need a single black hole distribution group in your organization.

Until next time…

As always, if there are items you would like me to talk about, please drop me a line and let me know!

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