There is a worldwide issue reported today with Trend Micro Hosted Email Security.
Most users of this service are seeing inbound emails rejected. Some are still receiving email but senders are getting rejection notices even though email has been delivered.
Other users are just not getting the emails.
Trend Micro are working to resolve this issue and expect it to be resolved today.
Once Trend Micro have resolved this issue then notification will appear at the end of this article.
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UPDATE 21:30 21st March 2013
This issue has been reported as resolved by Trend Micro