How to allow mail for same domain from different Exchange Server?

We have a single Exchange 2007 Server for somedomain.com, but there is another Exchange Server on same network which hosts an application which sends email with user@somedomain.com as the FROM and TO fields.
These emails are getting caught in our Content Filtering and I have already read up about not allowing the domain name as this is a classic way spammers try to send email. From what I can tell I should be using IP Allow List and add the IP of the other Server. Is that all I need to do? The content filtering bounce back looks something like below, and my question is which IP should I be entering into IP Allow List?

Delivery has failed to these recipients or distribution lists:
 
Fabio.Malerba@rolferenault.com.au
Your message wasn't delivered because of security policies. Microsoft Exchange will not try to redeliver this message for you. Please provide the following diagnostic text to your system administrator.
 
The following organization rejected your message: mail.rolfecarsales.com.au.
 
*********************************************
Diagnostic information for administrators:
 
Generating server: anotherdomain.com
 
user@somedomain.com
mail.somedomain.com #<mail.somedomain.com #5.7.1 smtp;550 5.7.1 Message rejected as spam by Content Filtering.> #SMTP#
 
Original message headers:
 
Received: from localhost.localdomain (172.16.1.1) by
 server.anotherdomain.com (172.16.1.98) with Microsoft SMTP Server id
 8.1.436.0; Tue, 16 Sep 2014 16:50:59 +1000
Date: Tue, 16 Sep 2014 06:50:48 +0000
From: <user@somedomain.com>
To: <user@somedomain.com>
******************************
LVL 6
FlippAsked:
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FlippAuthor Commented:
I am not trying to allow another server to send through my server. Lets say that my server is called MYSERVER and their server is called THEIRSERVER, and my mail domain is called MYMAILDOMAIN.COM.

THEIRSERVER sends email using USER@MYMAILDOMAIN.COM as FROM and TO address in mail message. When MYSERVER receives this mail message is passes through the normal processes, one of which is Content Filtering. How can I allow THEIRSERVER to send messages like this?

I am not prepared to add MYMAILDOMAIN.COM as a SafeSenderDomain due to spamming risk, so assume I can add via IP Allow List or something similar.
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hecgomrecCommented:
You don't have to.

First check if all your SMTP are listed:

Get-TransportConfig | Format-List InternalSMTPServers

If you are missing any just add your SMTP server to the internal list: (this will force the filter to ignore the IP)

Set-TransportConfig -InternalSMTPServers @{Add="10.0.1.10","10.0.1.11"}
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FlippAuthor Commented:
Thanks hecgomrec.

I am yet to implement your suggested changes, but will take a look very soon :)
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