IMAP Application needs more than 8 client connections to Exchange 2019 mailbox

Question

I'm trying to connect an IMAP based application to an Exchange 2019 mailbox. The application will create a bunch of IMAP connections to the same mailbox.

If it connects more than 8 times - the next connections are dropped.

I already tried to create a fitting ThrottlingPolicy and assigned it to that mailbox - but no success.

Then I tried some registry settings in HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ParametersSystem. and have set Maximum Allowed Sessions Per User to 2048.

But nothing helped!!

 here the IMAP log:

2024-03-22T10:31:24.425Z,0000000000000BE6,2,10.38.1.241:1993,10.38.1.242:18203,fireboard,44,31,30,authenticate,PLAIN,"R=""1 NO AUTHENTICATE failed."";Msg=""User:LegacyDn: /o=xy/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=01162ece3e8a437baa73ea8368264bcc-fireboard, RecipientType: UserMailbox, RecipientTypeDetails: UserMailbox, Selected Mailbox: Display Name: fireboard, Mailbox Guid: b9ff852e-0688-42d9-921c-2248e7ea9517, Database: e8f44cc6-69f3-4055-8e9a-b6f289b336f5, Location: ServerFqdn: EXCNLSKH01.xyz.de, ServerVersion: 1942128136, DatabaseName: DB01, HomePublicFolderDatabaseGuid: 00000000-0000-0000-0000-000000000000;Microsoft.Exchange.Data.Storage.TooManyObjectsOpenedException:Cannot open mailbox /o=xy/ou=Exchange Administrative Group (FYDIBOHF23SPDLT)/cn=Recipients/cn=01162ece3e8a437baa73ea8368264bcc-fireboard. --> Microsoft.Mapi.MapiExceptionSessionLimit:MapiExceptionSessionLimit: Unable to open message store. (hr=0x80040112, ec=1246)\nDiagnostic context:\n    Lid: 55847   EMSMDBPOO

Answer 1

Hello!

It migth be a restriction in an IMAP service.

If you have events in Application log from MSExchangeIS source with 9646 event id, you can see this restriction

You should use Set-imapSettings -MaxConnectionsPerUser cmdlet to fixed it

Answer 2

عندي نفس السؤال

Answer 3

عندي نفس السؤال

Answer 4

Hi fgc,

The IMAP based application you are referring to is a third party software and we can't confirm if he is able to connect to Exchange 2019 mailbox properly.

The log entry you've provided clearly indicates that you're hitting the ‘MapiExceptionSessionLimit’ error, with the error code ‘ec=1246’. This error occurs when a MAPI session limit is reached, preventing new connections from being established.

Considering you've already tried the standard approach of modifying the throttling policy and registry settings without success, there are a few other angles to explore:

1.After creating a new throttling policy with higher IMAP limits, ensure that you have assigned the policy to the mailbox and that enough time has passed for the changes to take effect.

2.Double-check the parameters of the throttling policy you've created. For example, make sure you've set the ‘RCAMaxConcurrency’ to a value that allows enough simultaneous connections. Here's an example of how to create a new throttling policy with increased IMAP limits using Exchange Management Shell:

New-ThrottlingPolicy -Name IncreasedIMAPConnections -RCAMaxConcurrency $null -EASMaxConcurrency $null -EWSMaxConcurrency $null -IMAPMaxConcurrency $null

Then assign it to the mailbox:

Set-Mailbox -Identity "fireboard" -ThrottlingPolicy IncreasedIMAPConnections

3.If you've made changes to the 'Maximum Allowed Sessions Per User' in the registry, make sure you've restarted the Exchange Information Store service for the changes to take effect.

4.Check the IMAP settings on the Exchange server to ensure there are no inherent limits set there that could override the throttling policy.

If the issue persists, please feel free to contact me, I’ll be happy to help you out!