Mike,
Both ebay and Paypal have had problems with this. Don't know if it applies to you, but if you haven't seen this, then you might want to check it out.
Look for this article:
***Protect Yourself From Spoof Emails***
Date: 08/14/03 Time: 10:22:37 AM PDT
http://www2.ebay.com/aw/marketing.shtml