Undeliverable Mail using Apple mail

I recently decided to transition from Outlook for Mac to Apple mail for obvious reasons - I don't like Microsoft. However, 50% of the emails, I am sending are being returned to me. These are the same people I have been emailing for years, some I am just replying to their message.

Why is this happening? Here is a string from the bounced email:

The original message was received at Thu, 6 Jun 2013 11:08:38 -0400

from [10.30.71.209]

*** ATTENTION ***

This email is being returned to you because the remote server would not

or could not accept the message. The registeredsite servers are just

reporting to you what happened and are not the source of the problem.

The address which was undeliverable is in the section labeled:

"----- The following addresses had permanent fatal errors -----".

The reason your mail is being returned to you is in the section labeled:

"----- Transcript of Session Follows -----".

This section describes the specific reason your e-mail could not be

delivered.

Please direct further questions regarding this message to your e-mail

administrator.

--Registeredsite Postmaster

----- The following addresses had permanent fatal errors -----

<ADVERTISING@EGRIPS.COM>

(reason: 554 5.7.1 Service unavailable; Client host [209.17.115.51] blocked using zen.spamhausdnsbl; http://www.spamhaus.org/query/bl?ip=209.17.115.51)

----- Transcript of session follows -----

... while talking to mail.mailroute.net.:

DATA

<<< 554 5.7.1 Service unavailable; Client host [209.17.115.51] blocked using zen.spamhausdnsbl; http://www.spamhaus.org/query/bl?ip=209.17.115.51

554 5.0.0 Service unavailable

<<< 554 5.5.1 Error: no valid recipients

Reporting-MTA: dns; atl4mhob13.myregisteredsite.com

Received-From-MTA: DNS; [10.30.71.209]

Arrival-Date: Thu, 6 Jun 2013 11:08:38 -0400

Final-Recipient: RFC822; ADVERTISING@EGRIPS.COM

Action: failed

Status: 5.7.1

Remote-MTA: DNS; mail.mailroute.net

Diagnostic-Code: SMTP; 554 5.7.1 Service unavailable; Client host [209.17.115.51] blocked using zen.spamhausdnsbl; http://www.spamhaus.org/query/bl?ip=209.17.115.51

Last-Attempt-Date: Thu, 6 Jun 2013 11:08:39 -0400

Any light you can shed would be appreciated!