I've been googling around for this, and much to my surprise I've hardly found anything on it. Most 501 errors the search engine found any info on were other kinds of 501s...I guess this type is a little more elusive? In any case, here's what I'm dealing with:

Your message did not reach some or all of the intended recipients.

Subject: torque calibrartion
Sent: 5/16/2006 6:13 PM

The following recipient(s) could not be reached:

<flastname@domain.com> on 5/16/2006 6:13 PM
501 <<flastname@domain.com>>: missing or malformed local part
The email address is typed correctly. Any idea what this means?

Note that both sender and recepient are using accounts on the same domain, same Exchange server.

I just sent a test email to that address myself (I'm on a totally different domain and server), and nothing bounced back.