We recently setup a 2010 server and found that a number of users could receive but not transmit mail using their iphone. We tested with our iphone in the office and had no problem.
Activesync using Android phones also worked without a problem
Most of the user had never upgraded the ios on their phones, or did not have SSL enabled in there mail accounts which 2010 was forcing. Once the ios was updated and ssl switched on all started flowing.
From this issue the bug appears to be specific to iPhone 4 versions.
Apples support topic-