External mass mail to organization with a hidden email address

Hi there,

We are using an external mass mailer service to send our company newsletter to every employee in our organization. This causes a lot of stress on our 2010 Exchange box. To make things even more difficult, the mass mailer uses random mail servers, so we can't pointpoint one external sender address. 

Here's what we need to accomplish:

The newsletter should only be sent to a single account in the organization instead of mailing to hundreds of internal accounts. The newsletter needs to be distributed to every employee inside the Exchange organization with a hidden sender ("from").

I was searching high and low but couldn't find the ideal solution.

Any help is highly appreciated!

Thanks

Dennis