Missing Text in body of email

I know this question has been asked before. Sorry I will have to ask it again, because this issue is giving me the biggest headache. My Client is using Outlook 2003 as his email client. He has a client he does business with who uses Lotus Notes. When my client sends a email to his client the email looks fine. Now when his client using Lotus Notes replies back to the original email he sent. My client receives the email with the message body missing. The signature block is there but the message body is missing. I've done some research on the internet and found that Notes RTF and Outlook RTF are not compatible. So emails should be sent in Plain Text format. On my clients email client I set Outlook to send as Plain Text Format and I unchecked Use word as the email editor.

This problem still exists. I can't get my client to ask his client to change Lotus Notes so he can send as Plain text also. There is no known fix for this problem on Microsoft's site or IBM. My client is convinced its our problem because the guy works at a bank and he does business with hundreds of users. So its got to be us. I'm planning to upgrade my client to Office 2007 to see if this fixes the problem. Has anyone run into this problem and if so, how in the world can it be resolved. I'm ready to quit behind this issue... My client has Office 2003 Pro SP3.