It has come to my attention that the checkrcpt patch for qmail seems to have disappeared from earth. So here is my copy of the patch, reposted along with some other helpful files.
Note: This is for qmail+vpopmail only. You should be running supervise, and a "run" file is supplied that has the appropriate config for this patch.
No warranty expressed, offerred, blah blah blah. Use it at your own risk.