In the programs I've seen, mail in the "Sent" box is dead. It will not try to resend. It's the "Out Box" where mail waiting to be sent sits. If no connection is made, an item in the Out Box may be retained, and your system may attempt to send it again later.

With at least some html mail, the "Out Box" is on the server and not visible to you. No reputable service will try to send from the outbox more than a very few times before it "drops" the message.

As long as a connection is made, the item normally is sent to "somewhere," and your copy of it is moved to the Sent box. Your machine does not attempt to resend once it's moved there.

Any server on the web that receives the mail item and is unable to immediately pass it on may wait and retry, but normally will try only a very few times. It usually will send you a delivery failure notice only when it "gives up" and deletes (drops) the message.

If the message arrives at a valid service where it looks like the addressee might "live," that service may hold it in a "pending" folder and may retry sending to the individual recipient a few times. That "end-point" server could send you a "nondelivery" notice for each try. Usually it's a "three strikes and out," or something comparably brief even there, but it's at the option of the service at the other end. Most services will send you a nondelivery notice ONLY when they decide they can't deliver the message, and ONLY ONCE on their last attempt.

The service at the receiving end may occasionally send you a delivery delayed notice, if they're having problems with their system. If they're unable to look up the address due to a system muckup, they might do this even for an incorrect address; but they should not send more than once. A "delivery delayed" notice could quite reasonably be followed by ONE "delivery failed" notice.

Normally, if you receive more than ONE nondelivery notice on a single message, you may SAFELY ASSUME that only the first one is valid, and the rest are fakes, phonys, frauds, phishing, ... or in other words junk. You probably are just being SPAMMED.

Your recourses are:

A Good AV program with CURRENT definitions, and scheduled scans.
Ad-Aware and Spybot at least weekly
(Possibly also Microsoft AntiSpyware and Microsoft Malware Remover, if your system qualifies)
Current Security Patches on your OS and Browser (Automatic updates recommended if available for your stuff)

TOTAL ABSENCE OF CURIOSITY About ANYTHING that looks suspicious in your email (don't even think about clicking)

Rapid Reflex Response using the DELETE button.

Faith that it will all not matter much when the sun eventually goes supernova may help, if you're so inclined. Spammers do, usually, give up eventually.

John