We just converted 4 of our AD security groups to mail-enabled. These groups grant access to different areas of an application and we occasionally need to notify the users of planned outages, unexpected outages, etc, of that application. This is why we converted those groups to mail enabled.
However, some of our users are in multiple groups. I'd like to send one email to all 4 of these groups to notify the users. If I do this, how many emails will the members who exist in >1 group receive?
For example:
Member A is in group 1 and group 2. I send an email to where both groups are in the To field. How many emails will Member A receive?
I would typically test this myself but creating a new security group requires a CR and whatnot from the administration team, so I'm hoping someone can just give me the answer.
As part of the message processing sequence, Exchange expands all the recipients including group members and discards duplicates before delivering the message. https://learn.microsoft.com/en-us/exchange/mail-flow/mail-routing/recipient-resolution?view=exchserver-2019
So, if someone is a member of multiple groups in the recipient fields, they only get the message once. This may not work 100% if there is some message originating from an external source that's using weird combinations of recipients, groups and BCC, but Exchange will almost always prevent multiple delivery of the same message to a recipient.
Here's a handy diagram of the mail flow sequence, at least for on-prem Exchange - EXO will be substantially similar for message processing: https://learn.microsoft.com/en-us/exchange/mail-flow/mail-flow?view=exchserver-2019
And you can test it by putting your name - or a colleague's - into both the To and CC fields. The message will only be delivered once. TBH, if it's a major hassle to do change control for testing out stuff in your production environment, you should at least have access to a test environment with less onerous change controls for this kind of thing - not least to figure out changes in advance of implementing them in production!
When you send an email to multiple mail-enabled security groups, each user will receive only one email, even if they are members of multiple groups. So, in your example, Member A who is in both Group 1 and Group 2 will receive just one email.
This is because the email system de-duplicates the recipients, ensuring that each user gets only one copy of the email, regardless of how many groups they belong to. This should make your notifications more efficient and prevent users from being overwhelmed with multiple emails for the same notification.