Strange thing is I had the same problem on the first M8 I had but somehow was able to get it to work. I had to exchange the phone for another issue and I can't get it to sync all email on the new phone. I only have about 250 emails and I've tried changing how much of an email to sync (2k, 5k, header only, etc.).
Has anyone else had this problem and knows how to fix it?
I set up my exchange email account and when I select to sync all email, the phone will show updating then it will start to show the number of emails being synced over the total but it gets to around 15 and starts all over. If I leave it, it will continue to try and sync until the battery dies. If I change it to any other amount like 30 days, the phone syncs fine. If I then change it back to all, same problem as above.