If you have a synced distributiongroup from your on-premise environment to your office 365 environment and you can not send from an external account to your distribution group, it might be because only authenticated users” are able to send to this address.

Changing this inside your Exchange O365 portal  will result in:

The action ‘Set-DistributionGroup’, ‘RequireSenderAuthenticationEnabled’, can’t be performed on the object ‘DG_PRTG.Notificatie’ because the object is being synchronized from your on-premises organization. This action should be performed on the object in your on-premises organization.

You can change this by changing your local distribution group.You have to change the“msExchRequireAuthToSendTo” parameter and set it to FALSE (use ADSIEDIT)