Natty,
When the mail is returned, there is usually an explanation as to why.
What was contained in the "return" message?
Be cautious of returned mail. Many "returned" emails are not such at all, but cleverly redirected "infected" mail.
They are designed to trick the receiver into thinking they had an email returned but in fact it was intentionally sent.
Before I can tell you "why" the legitimate email was returned, I need to know what was in the content of the message as to why they were returned.
Some reasons may be: server problems, ISP bandwidth and routing problems, closed accounts etc.
Techie