If you try to use a Telnet connection to try to connect to the mailbox by using POP or IMAP, you receive the following 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 Computer: Servername.contoso.com Description: 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="http://schemas.microsoft.com/win/2004/08/events/event"> <System> <Provider Name="MSExchangeIS" /> <EventID Qualifiers="49158">9646</EventID> <Level>2</Level> <Task>6</Task> <Keywords>0x80000000000000</Keywords> <TimeCreated SystemTime="SystemTime" /> <EventRecordID>126268</EventRecordID> <Channel>Application</Channel> <Computer>Servername.contoso.com</Computer> <Security /> </System> <EventData> <Data>/o=First Organization/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn= E-mailUser </Data> <Data>16</Data> <Data>session</Data> <Binary>070000005B444941475F4354585D000016000000FFE83A00000000000002080000003A67F01FFE000000</Binary> </EventData> </Event>
Note 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, 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.
Important 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, click the following article number to view the article in the Microsoft Knowledge Base:
322756 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 this is not the case, then increase the limit that your organization requires for non-MAPI sessions. To do this, follow these steps:
On the server that is running the Exchange Server 2010 Mailbox role, click Start, click Run, type regedit, and then click OK.
Locate and then click the following key in the registry:
On the Edit menu, point to New, and then click DWORD Value.
Type Maximum Allowed Service Sessions Per User, and then press ENTER.
On the Edit menu, click Modify.
Type the decimal value that specifies the number of sessions that you want to use, and then click OK.
Exit Registry Editor.
Note 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, visit the following TechNet Web pages: