Hi Leunam,
Based on my experience, the kind of issue may be caused if the format in which messages are sent to an external domain can't be interpreted.
In fact, messages that are sent from Exchange Online use the Transport Neutral Encapsulation Format (TNEF) format. Microsoft Outlook is one of the few e-mail clients that support TNEF-encoded messages, and some third-part message systems can’t support this format.
In this situation, to resolve this, you may disable TNEF encoding for all messages sent to all domains outside your organization or a specific domain outside your organization by using Windows PowerShell. For more detailed information, please refer to this article:
http://help.outlook.com/en-us/140/gg263346.aspx
Thanks,
Jason Jiang