Do Lync Conference Policies Apply to Distribution Groups?

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Generally speaking, Conference Policies within Lync Server exist to control functionalities and features when Lync modalities – such as IM, Audio, Video, App Sharing, etc – involve more than two participants.  There are some Conference Policy settings that do control peer-to-peer functionalities, but we’ll save that discussion for another day.  Any time a Lync modality reaches three or more participants, the traffic is then anchored on the appropriate multipoint control unit (MCU) on the Lync front-end server of the conference originator’s account.  A question was recently posed by someone about whether the Conference Policies would still apply when the Lync modality was directed at an Exchange Distribution Group.  My initial thought was “absolutely”, but I set out to test this to be sure.
To start off I changed my Global Conference Policy so that the maximum meeting size was 2.   Note: You cannot set the maximum meeting size to 1 as Lync Control Panel and/or Lync Management Powershell will return an error, even though the TechNet documentation states the maxmeetingsize value could be 1.

After allowing CMS replication to complete, I logged in to three workstations with my test accounts.  User A (Ethel) sent out an IM to a distribution group that included herself and two additional people, User B (Johnny) and User C (John):


User B sees the toast notification and clicks to accept and is joined into the IM conference:

User C sees the toast notification and clicks to accept but is not joined into the IM conference:

As expected, the Conference Policy did apply and restricted user C from joining the meeting.  The client GUI displays an error message and the Lync client tracing logs indicate the error as well:

ms-diagnostics: 3100;reason=”Maximum participant count for this meeting has been exceeded.”
As expected, the Conference Policy does apply when initiating modalities to Exchange Distribution Groups through the Lync client.  With this in mind it is important – to paraphrase the bears in the fairy tale “The Story of the Three Bears” – that your configured value for the maximum meeting size is “not too big”, or “not too small”, but “juuuuust right”.