No, it isn't coming from the registrar, I'm pretty sure that it is spam. The domain name in question is a throwaway one that I have, it is far from what you would consider a "premium" name.
Here is just one of the messages I've received:
from <xxxxxx@upgradedaddresses.com>
to "myemail@gmail.com" <myemail@gmail.com>
signed-by upgradedaddresses.com
A couple of days ago I emailed you about the domain name
<mydomainname>.com . Since you own the .net or .org version of
the domain, I thought you'd want the preferred '.com' version.
If you're interested, please let me know or use the secure link
below to buy the domain as soon as possible otherwise I'll make
other plans with the domain.
Here's the Secure Link:
http://upgradedaddresses.com/buy.php?domain=<mydomainname>.com
Also, Here's the email I sent you a couple of days ago
in case you missed it....