According to HTTP standards, all response codes in the 200 range mean "Success. The command was received, understood, and accepted." A Response Code of 204 only means that their RESPONSE had no content returned in it. If there was some error, you would get a 400 or 500 type response code. Therefore, your first test with the EAIF emulator looks successful.
Here are some examples from successful Nokia MMSC EAIF Emulator logs:
Terminating Application Log
<I> Received response:
<I> HTTP/1.1 204 No Content
<I> X-NOKIA-MMSC-VERSION: 1.1
<I> Response successfully validated
I understand how the terms can be confusing. This is because the EAIF emulator works opposite of what your Application does, but should always be set-up according to the classification of your application. If your Application will be RECEIVING mms messages, then it is a "Terminating" Application and the MMSC EAIF Emulator should be set-up for testing "Terminating". If your Application will be SENDING mms messages, then it is "Originating" and the MMSC EAIF Emulator should be set-up to test "Originating".
For more information and examples, refer to
Your application sounds like an "Orginating" Application because it will create the MMS message and send it out. So, when you use the EAIF Emulator, set it to testing for "Application"->"Originating".