When the Possible Delivery feature is enabled, AsyncOS treats any message that times-out after the body of the message is delivered, but before recipient host acknowledges receipt of the message, as a “possible delivery.” This functionality prevents recipients from receiving multiple copies of a message if continuous errors at their recipient host prevent acknowledgement of receipt. AsyncOS logs this recipient as a possible delivery in the mail logs and counts the message as completed. It is recommended that the Possible Delivery feature remains enabled.

You can change the behaviour if you log in the IronPort via SSH or
Telnet und use the command:
deliveryconfig->setup and then the second question you are asked is
about using it.

Guys , one of the system we are supporting , got a strange situation emails sent to same domain in mail_logs have these :
Sun Mar 23 13:04:34 2008 Info: MID 83361 RID [0] Response
'possible delivery'
and the same domain giving us in log these :
Sun Mar 23 14:48:27 2008 Info: MID 83936 RID [0] Response 'Thanks'
the one with possible delivery is not delivered to end user and the one with Thank's is delivered to end user , can somebody explain why ?