Q: ipad limits on activesync folder size

i have a senior user with a brand new 32GB ipad 3g, active sync set to an exchange 2010 server. This user has approx 15-20K messages in his sent, going back to 2006. On the device, he has no limits set for manual sync on his email. What is happening is that after the initial sync of his SENT folder, it stops syncing, usually 2 or 3 days. Are there any limits on folder size, or message count, in iOS. There is plenty of space on the iPad hardware.