Issue: Out of office is not working for external users in Exchange Server 2010 and it also applies to Exchange 2007 latest service pack.
External out of office was already allowed under remote domain in my case.
As an initial troubleshooting I tracked the out of office message and found “No Sender” in return path, then checked with my gateway team and they confirmed that messages are getting dropped at gateway end as it is coming from no sender.
Then I did a little search over the internet and found this is the default behavior of Exchange server 2010. Please read the below article.
We are using IronPort gateway as smarthost which supports external out of office messages and it was already enabled but still no luck.
In my case Setting Non-Mime character to “none” under message format of remote domain properties made the trick.
Hope it helps!!!