A vs MX, what do spammers prefer nowadays?

[tony]

Closing down the mailserver at the A-site and relying only on the MX-server 
gave me some weird (in the "I'm feeling all alone now that I don't get any 
spam"-way) results; and I can't really explain the results as I don't have 
enough data to say anything for sure, and instead of spending a cpl of weeks 
collecting enough data from the few servers that I can play around with as I 
please (without possibly upseting anyone) I thought I'd ask you all... 

What are spammers mostly targeting nowadays; do they ignore MXs, do they 
target MXs based on priority and do they probe the different options (As and 
all MXs) to find what works best? 


   /Tony