getting returned mail not from who was sent

[Fank]

A couple times recently I sent an email from my .mac account, from my imac, and received this response:

- - -

This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

******trungfa2@hotmail.com


Reporting-MTA: dns;COL0-MC2-F8.Col0.hotmail.com
Received-From-MTA: dns;cdptpa-imtalb.mail.rr.com
Arrival-Date: Mon, 28 Jan 2013 22:57:19 -0800

Final-Recipient: rfc822;trungfa2@hotmail.com
Action: failed
Status: 5.2.2
Diagnostic-Code: smtp;552 5.2.2 This message is larger than the current system limit or the recipient's mailbox is full. Create a shorter message body or remove attachments and try sending it again.

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THE PROBLEM is, I didn't send any email to that recipient, never ever.

So I reply to 'postmaster@hotmail.com' and of course it gets kicked back to me, but with this message:

postmaster@hotmail.com is an unmonitored account. If you need assistance, please consult our help pages or visit http://postmaster.live.com.

Thank You,
Windows Live Hotmail Staff

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Any ideas on what is going on? I don't feel like emailing trungfa2@hotmail.com as I have no idea who they are, but think it's strange that it is possibly trying to send an email I sent, to someone else, to this strange email address.