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 text to send to clients to add
certain events to their calendars. We tested the invites on our
Outlook 2010, Android, Iphone, gmail. They all worked for us.

We use Java mail to send the invites "javax.mail.Transport.send()".

What could cause these messages to be marked as poison. Could we be
generating the emails incorrectly, or with some bad characters?

Is it an issue with our SMTP server sending the emails, or more likely
with the content of the email we generate?


Specs of the Exchange Server on the Client side detecting our emails
as poison.
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