Overview
When sending to a mail-enabled group email address you may get this Non-Delivery Report (NDR) message, also known as a bounce message.
Delivery has failed to these recipients or groups:
Your message couldn't be delivered because the group you sent to isn't ready to accept messages. Before you can send mail to the group, it needs to have sender restrictions or message approval set.
Ask the owner of the group to set message approval (also called moderation) or sender restrictions for the group, then try resending your message.
For more information (including how to find the owner of a group), see
DSN 5.7.1 Errors in Exchange Online and Office 365(opens in a new tab).
Diagnostic information for administrators:
Generating server: MN2PR04MB6848.namprd04.prod.outlook.com
Remote server returned '550 5.7.125 RESOLVER.GRP.Blocked.NeedsSenderRestrictions; DL expansion needs sender restrictions or message approval configured'
Senders may also see these warning messages in Outlook, although it is not indicative of delivery issues with the group.
Outlook Web: "This message will be sent to at least 1,000 recipients."
Outlook Desktop: "contains at least 1000 recipients"
Resolution
As indicated by this article, DSN 5.7.1 Errors in Exchange Online and Office 365(opens in a new tab), the mail-enabled group will need to have moderation enabled. This will require contacting your local IT support team for assistance.