We are doing some analytics on our open rates grouped by domain and have uncovered some strange data. I had assumed that the gmail.com and googlemail.com domains behaved exactly the same, however, here are the stats we have for a recent week period:

sent = 299308
opened = 34353
open rate = 11.5%

sent = 45975
opened = 13019
open rate = 28.3%

They were sent the same campaigns at the same times etc.

This seems like a very big difference in open rate between the two domains and which I assume is too big to be accounted for as just statistical noise. Does nayone have any ideas as to why we are seeing this effect?