The group <group name> only accepts messages from people in its organization or on its allowed senders list, and your email address isn’t on the list.
More Info for Email Admins
Status code: 550 5.7.133
This error occurs when the distribution group, security group, or Office 365 group is configured to accept messages only from authenticated senders (senders in the same organization or those added to the group's allowed senders list).
To fix the issue, the recipient's email admin or the group owner must add the sender's email address to the group's allowed senders list or change the group's delivery management setting to accept messages from senders inside and outside of the organization.
Usually this issue can only be fixed by the recipient's email admin or the group owner.
For more information and steps to fix this error, see Fix email delivery issues for error code 5.7.133 in Office 365.
Assuming that the Office365 to On-Premise Exchange connect have been validated and functional, this command would fix the problem.
Set-DistributionGroup $groupName -RequireSenderAuthenticationEnabled $false