Error message
===============
Application Event: Mapi session "/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=E-mailUser" exceeded the maximum of 16 objects of type "session".
IE Error: A problem occurred while you were trying to use your mailbox
IE Error: 尝试使用邮箱时出现问题
Cause
===============
This issue occurs because the Exchange 2010 Store limits the number of non-MAPI sessions that other services generate, such as IMAP, POP, Outlook Web Access, Microsoft Exchange ActiveSync (EAS), or Exchange Web Services (EWS). By default, sessions per user are limited to 32, and service sessions per user are limited to 16.
Solution
===============
1.On the server that is running the Exchange Server 2010 Mailbox role, click Start, click Run, type regedit, and then click OK.
2.Locate and then click the following key in the registry:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem
3.On the Edit menu, point to New, and then click DWORD Value.
4.Type Maximum Allowed Service Sessions Per User, and then press ENTER.
5.On the Edit menu, click Modify.
6.Type the decimal value that specifies the number of sessions that you want to use, and then click OK.
7.Exit Registry Editor.
Reference
===============
You cannot connect to an Exchange Server 2010 mailbox from Outlook Web Access
http://support.microsoft.com/kb/980049/en-us