Ahhh you would be better of saying so from the start.
Within SMTP you have options which an also fool people as to recipents.
There are lots of ways of fooling people which the spam emailers work upon.
For example when you send an email the protocol allows you to specify a return address and a presentation name. These are the things most of the abuse comes from.
You can send an email saying your
[email protected] with a reply address of
[email protected] and a presentation name of NNNNNN
Anyone receiving the email will see NNN and possibly the
[email protected] address and may not notice the address has changed when they try to reply.
It is incredibly easy to create the above scenario - which is why email can be so easily abused.
If your email application gives you the opportunity to look at the headers you can easily spot whats going on as the various fields are obvious - SMTP is a highly readable format to the average person.