Did you set the MSGFLAG_RESEND flag in PR_MESSAGE_FLAGS?
FromMSDN:
Resending an Undelivered Message
A transport provider sends a nondelivery report (NDR) when it cannot
successfully deliver a message that you have submitted. It is up to the
client whether or not users can attempt to resend these undelivered
messages. If you support resending messages, you can either use a form
provided by MAPI or implement your own. The MAPI form displays the names of
the failed recipients and the reason for the delivery failure, if possible,
and includes a button that, when selected, allows a user to resend the
message.
When a resent message is received, it should look exactly like the original
message. The recipient should be unable to differentiate between a message
that was delivered on its first attempt at transmission or a subsequent
attempt. Replies on this message should work exactly as if the message had
been sent successfully the first time.
To resend an undelivered message
Call IMAPIFolder::CreateMessage to create a new message.
Copy all of the properties from the original message, excluding
PR_MESSAGE_RECIPIENTS, the PR_SENDER properties, and the
PR_SENT_REPRESENTING properties. Make the following property modifications:
Set PR_MESSAGE_CLASS to the report's PR_ORIG_MESSAGE_CLASS property.
Set the MSGSTATUS_RESEND flag in the PR_MESSAGE_FLAGS property.
Set PR_ORIGINAL_ENTRYID to the original message's PR_ENTRYID property.
For each recipient, set MAPI_SUBMITTED in the PR_RECIPIENT_TYPE property.
Duplicate each failed recipient. Change the PR_RECIPIENT_TYPE property for
the duplicated recipient to MAPI_P1. Therefore, for each failed recipient
there are now two entries in the recipient table: one with PR_RECIPIENT_TYPE
set to its original value and the other with PR_RECIPIENT_TYPE set to
MAPI_P1.
Call ScCreateConversationIndex to set up conversation tracking if desired.
Call the new message's IMessage::ModifyRecipients method to update the
recipient list.
Call IMessage::SubmitMessage to save and send the new message.
Dmitry Streblechenko (MVP)
http://www.dimastr.com/
OutlookSpy - Outlook, CDO
and MAPI Developer Tool