source mail server IP differs from MX IP: potential blacklist ramifications?

Hi there, 

My eyes have been swimming from all the reading I did this weekend, and this 
last issue was when I was a couple short of a six-pack...both literally and 
figuratively. 

We are dealing with SPAM (who isn't). But have no real solution in place right 
now. One particularly attractive option: not making it my problem. 

My company recently merged with another company, which has an IT staff of 12 to 
my one with apprentice. They have Saphos Puremessage for Unix in place, and 
offered to help. 

The idea seems simple enough.
I modify my MX to their perimeter X.X.X.X server.
They filter and forward to our server based on a table entry similar to DNS.

However, all my sent messages will originate from my Y.Y.Y.Y ip (I'd rather not 
have it go back through their system)

So, does this pose a potential issue regarding blacklists. I *believe* one 
method of determining SPAM sources is a comparison of source IP to public 
record (MX)? Or am I really way off on the idea of Reverse DNS?

Thanks for the insights! Dan

Other related posts: