Windows Phone 7.5 users are reporting some quirks with how it interacts with Exchange 2003 servers. Firstly, when replying to an email using an Exchange 2003 account on a Windows Phone, the original message does not get included in the reply. What the recipient sees instead is just the header information from the original message (pictured above).
Another issue identified by Exchange 2003 users involves forwarded emails. When forwarding a message to someone, the original content gets sent to the recipient as an attachment (pictured below).
It's unclear why exactly this is happening, but it's certainly causing people frustration. It's likely due legacy technology Exchange 2003 , which has since seen two newer iterations, Exchange 2007 and the latest, 2010. Still, given that Exchange 2003 is clearly outdated and that it behooves companies to upgrade, the fact of the matter is that a brand new Microsoft product should interact flawlessly with another Microsoft product. To add some fuel to the already justifiable fire, let's consider that Microsoft's competitors, namely Android and iPhone, do not have the same bugs. Microsoft, you have some explaining to do.
NOTE: One commenter on the user voice page says that they are using Exchange 2003 SP2 and they are not experiencing the issue. The tests I ran where I did have problems were on the same version of Exchange, so perhaps there is some update out there that fixes this.
The Exchange ActiveSync Client Comparison Table on Microsoft TechNet does not list anything prior to Exchange 2003 SP2, so if you have WP7 working with anything older, then a hearty congratulations to you.
ANOTHER NOTE: This issue is being reported in some places as an Outlook issue, which is not the case. The problem here has something to do with how the phone OS itself and the Exchange server communicate, which does not involve an email client like Outlook. Though to be fair, when setting up an Exchange account in WP7, the option you choose is called "Outlook."
Source: Microsoft (Thanks for the tip, Mike!)