External members of a universal distribution list are not able to receive emails

Hi everyone,

I've an issue. I've created a universal distribution list on local AD. Purpose was to create a group which should have external members (our vendors) and internal members both. For creating external members, I've created 'Contacts' in local Active directory and added them to the respective distribution list.

Now when we send an email to this distribution group, only our internal members are able to receive the messages. External members are not able to receive any emails. Please note that this group is set for 'senders inside and outside of my organization'. apart from this, I only have the access on local Active directory where I can make changes using ADSI edit and AD console. Then I use DirSync (Forefront identity manager) to sync the changes to Azure 0ffice 365. I don't have access to office 365 admin portal.

Please...