How to allow external senders to a Matter group email?
Each Matter has its own email address. You can view the email address in the Matter's Dashboard in the "Matter Details" Tile.
By default Microsoft doesn't allow external senders to group emails.
To change that setting please follow these steps:
Select the Matter group in Outlook, and go to Settings:
Make sure to select the option to "Follow in inbox", this will allow external sender to send emails to the Matter group email: