Scenario
You are sending emails to users in Microsoft 365 groups, using Server-Side configuration. You notice that recipients are receiving two copies of emails.
Reason
This is a known issue with Microsoft.
When an email is sent to users in a group, the group mailing address is split into each user's individual address during processing. This means that two versions of the email are created: the original email, and an individual version for each recipient email address. Exclaimer processes a group email by using the individual versions and sending them to each address in the group. The original email is discarded or 'dropped'.
For Microsoft 365 groups, the original email is not dropped when the group is split up into the individual recipient addresses. Instead it is also sent to Exclaimer for processing. This means that Exclaimer processes both versions, and sends them back to the Microsoft tenant. The Microsoft tenant then sends both the processed individual emails and the group email, so users receive two copies of the same email.
Resolution
Microsoft considers this expected behaviour. There is no resolution and Microsoft do not plan to change this.
We recommend that you do not use Microsoft 365 groups; instead, use a distribution list.