cactus
Normally messages are kept in a queue for a while (24 hours most of the time)
qmail will keep trying for 7 days before advising the original sender that the message cannot be delivered.
Many systems use qmail, not just sme server, so you won't reliably know that a message was never delivered until after 7 days, although some mail servers are set for one or two days.
edit
I'd also add that if the external mail server you are sending to is down, then your sme server will generate this message after approx 10 hours
This is an automatically generated Delivery Status Notification.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
Delivery to the following recipients has been delayed.
<recipient@externaldomain.com>
The reason for the problem:
4.4.2 - Bad connection 'timeout'