Let's start with this tip:
"If you are asking for help to troubleshoot a computer-related problem, please be sure to include all the necessary information (ie: operating system, model number, hardware, software, etc) that will help others identify your problem for a speedy resolution."

The OS in use is very important since any DOS based Windows can quickly deplete the 64 thousand byte resource pool and you get this error. I ran into this years ago and my workaround was to send 10 to 20 messages, then restart the application. No fix is available for this issue! And there will never be one. (Just trying to be clear on this point.)

As to looking at your message, it's widely discussed, but given what's not in your post, I can't narrow the results down for you. You have to research more with THIS GOOGLE and use the Discussion Groups link as well.

While I know how to finally solve the issue, not everyone can program in Visual Basic (the easy way) or another language to get out from underneath the resource eating and slow Office applications. Our final solution was a custom VB app which did the merge and mail.

Bob