Fix the issue To fix this, add the sender to the group's list of allowed senders. Go to the Groups page. Choose the group, ...


Fix the issue
To fix this, add the sender to the group's list of allowed senders.
  1. Go to the Groups page.
  2. Choose the group, and then choose Edit.
  3. Choose delivery management, then Add.

For detailed instructions and additional tips, see Status code 5.7.124 in Exchange Online and Office 365.