I set up an open relay honeypot several months ago, but I noticed early on that if the spammers' test e-mails do not go through, they will abandon your SMTP server in a hurry.
Fortunately, nearlly all of the spammers that have sent test e-mails on my honeypot have followed a similar pattern: Namely, they always seem to include my IP address on the subject line. Usually something like this: SM:220.127.116.11 (SM for sendmail, I presume). Since this is the typical pattern, I have modified my honeypot program to let these types of e-mails through, and since I did this, my honeypot has been running non-stop night and day. I have dumped litterally millions of e-mails, and some of the same spammers have been using my honeypot for weeks or even months.