Thursday, September 06, 2007

Adding an X-header in an Exchange Routing Transport Agent in Exchange 2007

This week I’ve been looking at Transport Agents which are the replacement to Transport Event Sinks in Exchange 2007. After the initial struggle of finding some decent information to get going these Agents are surprising easy to code and have a bunch of powerful functions that are more readily available then they were when compared to SMTP transport Event Sinks.

Getting Start info

If you’re looking for info on getting start the first place is this simple sample . One piece of advice is before you look at installing a Transport Agent making sure you know how to disable or remove one first.

There are two types of Transport Agents Routing and SMTP which one you use really depends where in the Transport Pipeline you want to intercept the message. I choose a Routing Transport agent because I wanted to be able to add an X-header to a message regardless of whether the message arrived via SMTP or if it was sent locally on a Mailbox server (if you are looking to do this in a SMTP Transport agent there is a X-header Sample in the Exchange 2007 SDK). The Transport Architecture document describes the process of Mail routing via the Hub server.

Once you get over modifying the subject you may want to start modifying the rest of the message this is where it starts getting hard and the information starts to get a little scarcer. Depending on what type of message you’re dealing with you may need to cope with MIME and TNEF formats. The later being a slightly harder thing to deal with for serialized messages most of the time MIME should be enough. So I would recommend reading the following MIME Architecture document from the SDK. Being able to deal with a serialized message in the MimeDocument Class is pretty cool and very powerful. You should always be carefully that any modifications you do make to a message aren’t going to mean that message breaks the Email RFC’s.

Getting down to the code

There are a number of different events you can hook into with a Transport Agent the one I was interested in was the OnSubmittedMessage. The event occurs just after the message is submitted because I only want to modify the headers content conversion shouldn’t really be an issue. The code is rather simple it first looks to see if there is a Xheader within the message already with the Name i want to use. If there isn't it creates a new Text header and inserts it before the last header in the current header list in the root MIME document.

I’ve put a download of the Transport Agent here the the main code itself is the following Event Handler

public class XHeaderRoutingAgent : RoutingAgent
{
public XHeaderRoutingAgent()
{ base.OnSubmittedMessage += new SubmittedMessageEventHandler(XHeaderRoutingAgent_OnSubmittedMessage);
} void XHeaderRoutingAgent_OnSubmittedMessage(SubmittedMessageEventSource esEvtsource, QueuedMessageEventArgs qmQueuedMessage)
{
MimeDocument mdMimeDoc = qmQueuedMessage.MailItem.Message.MimeDocument;
HeaderList hlHeaderlist = mdMimeDoc.RootPart.Headers;
Header mhProcHeader = hlHeaderlist.FindFirst("X-MyProcess");
if (mhProcHeader == null)
{
MimeNode lhLasterHeader = hlHeaderlist.LastChild;
TextHeader nhNewHeader = new TextHeader("X-MyProcess", "Logged00");
hlHeaderlist.InsertBefore(nhNewHeader, lhLasterHeader);
}

}

}