Newbie needs some help ......

Yesterday I wrote a message requesting someone to host our
organization, Loners of America.

Within a few hours I received a very generous offer.  Then I sent a
reply to my own message using my mail clients "Reply To"  feature,
informing everyone that I had received an offer.  (I snipped most of
the original article).

Problem:   My reply was returned undelivered stating that it had
already been sent.....(???)

So I wrote a *new* original message with subject "RE:<the original
subject>"  but it too was returned undelivered with the same reason --
message had already been sent.....(????)

Well,  now I'm really confused.........  Is it the matching subject
that's being rejected -- even tho the second attempt was
"RE:<subject>"   ????

Russell