Summary
Based on the research found with Exchange Server 2010 and 2007's latest Service Pack, Microsoft has changed the behavior of automatic server based notifications, especially Out-of-Office (OOF) replies to comply with RFC 2298.
As stated in RFC 2298, Message Disposition Notification (MDN) messages should be sent with a blank sender. Out-of-Office reply are Message Disposition Notifications. This means that the HUB Server in the organization replaces the senders name with a blank one while transferring it to the internet.
In Trend Micro Email Security (TMEMS), for outbound messages, if the sender address is null, this will not show up in the Mail Tracking Logs. For inbound messages, it will show in the Mail Tracking Logs regardless of the sender, or even from a blank sender.
Out-Of-Office notification for remote users show with a blank sender. The message headers may look similar to the following image and this is by design according to RFC 2298:
Out-Of-Office notifications usually gets stuck in Smart Host or email security products. It is suggested to contact the vendor of the email security appliance to see if any configuration can be set to accept these Out-Of-Office messages.
For additional information, refer to this Microsoft community blog.