BTG> "qmail realrcpto"
BTG> "qmail goodrcptto"
BTG> "qmail badrcptto"

JdeBP> He wanted to check the envelope sender mailbox name, not
JdeBP> the envelope recipient mailbox names.

BTG> I misread the "existing-user" bit.

So did I, the first time that I read the message. I got partway through
composing a reply along the same lines as yours before I noticed.

BTG> JdBP's link would be much more helpful for the problem you
BTG> are trying to solve.

In general, one should always consult <URL:http://qmail.faqts.com./> before
posting a "qmail" question to Usenet or a mailing list.