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Processing BCC's in Exchange Transport Agents

One of the most important learning points when thinking about programing or designing Transport agents is to understand the different between a P1 (Delivery Envelope) and P2 (Display Envelope)

The P1 (Delivery Envelope) contains the recipient To and From information that is transmitted to the MTA server be it Exchange or any other SMTP server in the SMTP conversation and is the actual information that will be used for delivery of the message. Eg the Evaluating MTA will only examine the P1 recipients to determine what recipients a message would be delivered to.

The P2(Display Envelope) contains all the information that that the user will see when they receive the message this includes the From, To ,CC and Subject of a message. The important point to understand is that the MTA that is processing the message doesn’t look at these headers to determine whom to deliver the message to.

BCC’s

One of the things that confuses a lot of people when they look at messages via an API or within an Transport Agent is that most API’s include the ability to set and get BCC address’s but they will always appear blank when you attempt to read them for example when a message is passing through the transport pipeline. The simple explanation for this is if you have understood about P1 and P2 recipients is that BCC’s when a message is submitted become P1 recipients of the message for the first MTA to process but they are never added to the P2 envelope. From the MTA’s perspective because delivery of TO, CC or BCC are treated exactly the same these elements are merely display information and the MTA just treats these recipients as another RCPT TO entry. To detect BCC in a Transport Agent you can do two things the first is reconcile the P1 Recipients against the P2 Recipients if you find a P1 recipient that doesn’t have a matching P2 entry then you can assume that this is either a BCC or potentially an alternate or journal recipient. The other way is to look at the Microsoft.Exchange.Transport.RecipientP2Type http://msdn.microsoft.com/en-us/library/aa579279%28EXCHG.140%29.aspx EnvelopeRecipient.Properties Property this returns an Integer that tells you what type of recipient each P1 envelope entry is.

For example

foreach (EnvelopeRecipient envelopeRecp in QueuedMessage.MailItem.Recipients) {
object RecpType = null;
if (envelopeRecp.Properties.TryGetValue("Microsoft.Exchange.Transport.RecipientP2Type",out RecpType))
{
switch((Int32)RecpType){
case 1 : // Process To
break;
case 2 : // Process CC
break;
case 3 : // Process BCC
break;
}
}
}

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