Why don't I get a delivery failure message for a wrong recipient name?

I have script which sends email to the internet (john@domain.com). It is manually invoked, and if a user misspells the domain (john@doamin.com), they’ll get a delivery failure almost immediately.

However, if they misspell the recipient name (jhon@domain.com), then they don’t get a failure message at all.

How do I fix this?

(btw, I’m setting the principal, replyto, from, inetfrom and smtporiginator fields. And, mailtimeoutminutes=30 on the server.)