Hi IQA,
According to the description, I understand that you have followed the steps in the above article and create the external contact in Office 365 with the same email address of the DDG, however Office 365 users cannot send emails to this DDG at this time.
First I'd like to confirm whether you have configured the properties of the DDG by following the correct steps.
As you can find the information, On the Filter tab of the DDG Properties, select The following specific types, and then click to select the Users with Exchange mailboxes and the Users with external e-mail addresses check boxes.
Moreover, based on my experience, please click Mail Flow Settings tab and double click Message Delivery Restriction, Uncheck the Require that all senders are authenticated check box, click OK. After that, check to see if the issue persists.
Thanks,
Bourne Zhang