Group Won’t Accept Mail From ‘Outside’
KB ID 0001771 Problem Exchange has been this way for a long time here’s me explaining this very problem with older versions of Exchange. If you create a ‘Group’, be that a Distribution Group, or a ‘Microsoft 365′ Group, the default setting is to NOT ALLOW mail from anyone outside your organisation. If you attempt to send mail to that group you will see errors like these; Errors; 550 5.7.133...