Unable to Send emails

I have 2 IMac's. One IMac can send and receive emails fine. The other IMac cannot send emails. The emails show that they sent when you display "Sent", however the recipient does not receive the email.

We are networked through AirPort Utility.

I have checked all connections and ran diagnostics, and all are "green" and working fine.

I have double checked the mail settings and they match.

I have compared both terminals and they are setup identical in preferences.

I have logged off/on.

Still, unable to send emails. I can receive them fine, just cannot send.