Question : 550 Too Many Invalid Receipients

We do not have our own email server. We got through business.verizon.net but use our owndomainname.com
I have a user that keeps getting this error.  Emails have been addressed to one individual at a time. He's tried typing manually and not using a address book.  At first the error message would only happen when replying to an email.  After 4 weeks of trying to resolved this issue, the problem now happens when sending a new email.  I hope I provided enough information.

Your message cannot be delivered to the following recipients:

  Recipient address: [email protected]
  Reason: Rejection greeting returned by server.
  Diagnostic code: smtp;550 Too many invalid recipients
  Remote system: dns;external.clearchannel.com (TCP|206.46.252.115|43378|207.230.136.65|25) (Too many invalid recipients)

Reporting-MTA: dns;vms115.mailsrvcs.net (tcp-daemon)

Original-recipient: rfc822;cisqo@clearchannel.com
Final-recipient: rfc822;cisqo@clearchannel.com
Action: failed
Status: 5.0.0 (Rejection greeting returned by server.)
Remote-MTA: dns;external.clearchannel.com
 (TCP|206.46.252.115|43378|207.230.136.65|25) (Too many invalid recipients)
Diagnostic-code: smtp;550 Too many invalid recipients

Answer : 550 Too Many Invalid Receipients

I just checked connection to that server and your recipient (rcpt to: and mail from: are intentionnally changed):

250-smtp11.clearchannel.com
250-8BITMIME
250 SIZE 41943040
mail from: [email protected]
250 sender ok
rcpt to: cisqo _ @ _ clearchannel.com
250 recipient ok
data
354 go ahead
Subject: test

...
250 ok:  Message 76389008 accepted

Everything seems to be working from their server's side.

Now we see that: Reporting-MTA: dns;vms115.mailsrvcs.net (tcp-daemon)
It's not real DNS name, probably it's a fake name inside verizon. But sending IP is: 206.46.252.115, that resolves as vms115pub.verizon.net.
So I guess that you are sending email from vms115pub.verizon.net. to smtp11.clearchannel.com directly, without using 'business.verizon.net' mail server.

And also I see that  smtp11.clearchannel.com has strong antispam policy. Probably it stops receiving email from you after it has got some number of bad recipients in some period of time. I don't know which emails where bad, but surely not that you have mentioned above. It's better to ask [email protected]m why his server refuses emails from you.