I checked the zdi.com website. This is from the site:
ZD is an established electronic component distribution powerhouse with an emphasis on board level active semiconductor devices.
I don't know your SPAM prevention set-up, but if one of your users did not contact this site, I would think it was a piece of ***SPAM*** that got past your system.
You might want to monitor you mail log for a few days and see if the user responds.
I honestly don't think it was a hack attempt. Usually a hack attempt will try to login and they tend last for a while. I deliberately watched a kiddie-script try to get in on my server one night. It lasted over 3 hours trying all kinds of logins. I had just built the server so nothing was on it yet. They never got in.