Error: 421 4.4.1 Service unavailable, error interacting with downstream server

Hello,

I have exchange server 2010 sp2. Once a month we have a marketing letter that gets emailed. A small 250k word doc. They send 2 emails each to a group of 30-40 people.

I alsways get this error message a few hours after they send it, and i see it sitting in the mail queue.

Last Error: 421 4.4.1 Service unavailable, error interacting with downstream server

I believe i have my limits set high enought, but this error does not seem to be referencing any size or limit issues.

Thanks.