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You cannot connect to an Exchange Server 2010 mailbox from Outlook Web App

Original KB number:   980049


You can't use POP3 or IMAP4 to connect to an Exchange Server 2010 mailbox through Microsoft Office OWA. When you try to connect, you receive following error message:

A problem occurred while you were trying to use your mailbox.

If you select Show details, you see that the following exception causes the error:


If you try to use a Telnet connection to try to connect to the mailbox by using POP or IMAP, you receive this error message:

No Server

Unavailable 15

The server that is running Exchange Server 2010 logs an error in the Application log that resembles the following:

Log Name: Application
Source: MSExchangeIS
Date: dd/mm/yyyyhh:mm:ss
Event ID: 9646
Task Category: General
Level: Error
Keywords: Classic
User: N/A
Mapi session "/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=E-mailUser" exceeded the maximum of 16 objects of type "session".

Event Xml:

<Event xmlns="">
      <Provider Name="MSExchangeIS" />
      <EventID Qualifiers="49158">9646</EventID>
      <TimeCreated SystemTime="SystemTime" />
      <Security />
      <Data>/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=E-mailUser</Data>


The following changes do not resolve this issue:

  • You change the MaximumConnectionsPerUser value in Exchange Server 2010 for POPSettings or for IMAPSettings.
  • You change the Throttling policy.
  • You add the View Information Store Status permission to the Exchange 2010 Store.


This issue occurs because the Exchange 2010 Store limits the number of non-MAPI sessions that other services generate, such as IMAP, POP, OWA, 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.



This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, see How to back up and restore the registry in Windows.

To resolve this issue, first determine whether this situation is caused by a different issue. If not, then increase the limit that your organization requires for non-MAPI sessions. To do this, follow these steps:

  1. On the server that is running the Exchange Server 2010 Mailbox role, select Start, select Run, type regedit, and then select OK.

  2. Locate and then select the following key in the registry:

  3. On the Edit menu, point to New, and then select DWORD Value.

  4. Type Maximum Allowed Service Sessions Per User, and then press ENTER.

  5. On the Edit menu, select Modify.

  6. Type the decimal value that specifies the number of sessions that you want to use, and then select OK.

  7. Exit Registry Editor.


The registry value Maximum Allowed Service Sessions Per User affects all Mailbox Databases on the server and is not applied on a per-user basis. If you increase this value, server performance may be adversely affected. For example, doubling the number of service sessions from 16 to 32 could slow server performance.


For more information about how to set limits for sessions, see: