Spam - how does someone send an email to me using my own domain

I received an email form peterd@mydomain.com . How does a person send a mail from mydomain when he is not a user on that domain and more importantly WHY would he. Surely if I reply to it (Which I did not) he would not get it as it would bounce on my gateway. Whats the point.
GregopsgAsked:
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rowansmithCommented:
The how is actually quite simple.  An email message is really exactly the same (in principle) as a piece of posted mail.   It has a TO and it has a FROM the only thing that matters is the TO, the same as postal mail.  No one cares about the FROM and as with POSTAL mail I can put ANYTHING I want in the FROM address.

As for why - what's the point.  You are correct that you could not reply to it, but that is not the point.  The point is to deliver you a message with some content.  It is most likely that the content would have a link to a website.  Spammers do not want you sending email back to them, they want you taking whatever action is necessary from the contents of the SPAM.

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Neil RussellTechnical Development LeadCommented:
What you see as the senders address is nothing to do with where it came from, it is spoofed. When you reply it may or may not be to the address you see as the from address that it goes to, email headers can contain a Reply-to field that directs the reply to the address contained in that line.

WHY WOULD HE?
Good question!! Why be a spammer!?
If you find an answer let us all know! lol
GregopsgAuthor Commented:
Thanks Guys
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