I have a problem sending emails that happens sporadically. When I
send an email initially, the mail is returned undelivered. The
message is included below. If I resend the message from my Sent
folder; the email is delivered with no problems. Any help would be
greatly appreciated!

Bounce back message:

This is the mail system at host leviathan.cnchost.com.

I'm sorry to have to inform you that your message could not be
delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your
own text from the attached returned message.

The mail system

<D9225@IX.NETCOM.COM>: host mx3.earthlink.net[209.86.93.228] said: 550
D9225@ix.netcom.com...User unknown (in reply to RCPT TO command)

Details provided with bounce back message:

Reporting-MTA: dns; leviathan.cnchost.com
X-ConcentricHost-2-70--Relay-Queue-ID: 290952FF7
X-ConcentricHost-2-70--Relay-Sender: rfc822; education@softkeyinc.com
Arrival-Date: Sat, 9 Oct 2010 15:03:07 -0400 (EDT)

Final-Recipient: rfc822; D9225@IX.NETCOM.COM
Action: failed
Status: 5.0.0
Remote-MTA: dns; mx3.earthlink.net
Diagnostic-Code: smtp; 550 D9225@ix.netcom.com...User unknown