Herb Tyson said:
There are a number of very bad viruses, etc. going around... resulting in
much higher bounce rates than in the past. Basically, a bounce notice
indicates that a mail server somewhere has rejected your email, possibly
because it thinks your message contains a virus.
Frequently the bounced messages didn't actually come from you in the first
place. The current crop of virus/trojan/worm programs fake the From
addresses in the messages they send. When the target mailbox is full or
non-existent, the mail handler sends a notification back to the apparent
sender, who wasn't even involved in the transaction.
Some mail handlers will detect a virus attached to an incoming message and
send an announcement back to the apparent sender. Although that was
originally a helpful thing to do, it now does more harm than good and
should be discontinued. Even worse, some systems include the infected
attachment so the apparent sender, who may not have had a virus to begin
with, has a new opportunity to get one.