By default, Exchange only allows 250 messages to be open at any given time. There is a registry entry you can modify to change this limit on the Exchange server. Here is the Microsoft Technet article about the issue:
The object you want to modify is the objtMessage, and we recommend setting this limit to 600 to resolve this issue. If you have any appointments with more than 600 exceptions, you may need to set the limit higher to accomodate the number of exceptions you need to process.