We've been struggling to identify the reason why are emails are marked as "Poison Messages" for some of our clients. Our system generates iCalendar requests with some basic html to send to clients when to add certain events to their calendars. We tested the invites on our Outlook 2010, Android, Iphone, gmail. They all work. Recently we have had 2 clients says that our emails are being marked as "poison messages" by their exchange servers. What would cause this? Could we be generating the emails incorrectly? How can we even diagnose this? Specs of the Exchange Server Exchange 2007 Windows 2003 x64 R2 OS Hub Transport version 8.1 Build 240.6 (exact version of exchange build) Forefront Protection 2010 version 11.0.713.0 Symantec endpoint detection - AV is turned on but AntiSpam is turned off

You are on a version of Exchange 2007 that is if no longer supported must be very close to it - SP1. I would suggest that you start by upgrading Exchange to Exchange 2007 SP3 plus the latest rollup. However if your messages are going in to the poison queue then they are poorly formatted, and Exchange is treating them as bad. You will need to look at the format of the message. Testing with clients isn't really enough, you need to test with MTAs, so that would include things like Exchange, Notes, Groupwise, Zimbra, as well as POP3 based solutions. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources | In the UK? Hire Me.