Q: Are Deferred Action [Messages] stamped via the MAPI client, or via the server?
A: I'm not sure how that relates to
The first type is delayed send. This is the typical functionality you see when using Outlook to send email. This functionality is controlled by the MAPI property PR_DEFERRED_SEND_TIME. When this property is set, the the message is kept in the MSExchangeIS SendQ.
To end users, this means that their messages are visible in their Outbox. They can still double-click on them to open them, and will need to click "Send" for them to resubmitted for delivery.
For an administrator, they will need to log into the users mailbox to see the message in that users Outbox. Unless the mailbox is deleted and purged
The second type is deferred delivery (controlled by the MAPI property PR_DEFERRED_DELIVERY_TIME). The key difference is that the delay happens on the "delivery" side once transport "owns" the message. While the phrase "deferred delivery" might make you think that message is held on the destination server, it is in fact held in the queues