I was talking with a customer who was confused why they couldn't get their email to sync to their device even though they could still receive new email on their PC.
After a bit of investigation - This was due to Mailbox Quotas - the mechanism in Exchange to enforce limits on how much email you can store in your Mailbox. Essentially if you can’t manage your mail then you will be prevented from using it :)
When you setup Mailbox Quotas in Exchange there are 3 quotas you can set:
1) Issue a Warning
2) Prohibit Send
3) Prohibit Send and Receive
Somewhat strangely - Your Mobile device will stop syncing when you eclipse the second quota (Prohibit Send) - the reason is that the second quota prevents you from writing any new data to the Exchange store and the sync operation requires you to store the sync state in the mailbox.
Of course you can always free up some more space in your mailbox by going to Menu > Tools > Emtpy Deleted Items in Outlook Mobile.