Outlook connection issues on Exchange 2016 (ThrottlingPolicy)

After switching to Exchange 2016, I had a problem for a long time that sometimes MS Outlook 2016/2019 could not connect to Exchange Server 2016 on the same local network. If I let it run, it would connect after some time (estimated 10 minutes). This problem occurred if Outlook ran correctly, then closed and restarted repeatedly. Or if the user was switching between different networks (wired and wireless) with Outlook running. Eventually I found that this was probably caused by the RcaMaxConcurrency limit, which defaults to 40.

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How the Problem Manifests

After starting Outlook, the status bar will show the status as Disconnected.

When hovering over the Outlook icon, the message will display Microsoft Outlook cannot connect to the server.

If we want to view the detailed connection information, we can click Ctrl + right-click on the Outlook icon in the notification area (next to the clock) and choose Connection Status.

Identifying the Error

We can find the error in the MAPI HTTP log, but we probably need to know what to look for beforehand. This is the occurrence of the MaxConnectionsExceeded data. The log is typically located at the path

C:\Program Files\Microsoft\Exchange Server\V15\Logging\MapiHttp\Mailbox\

Its name is based on the date and time, for example MapiHttp_2020072806-6.LOG.

Example of a log entry with the rpc MaxConnectionsExceeded error.

2020-07-28T07:11:11.667Z,52c306ac-94e1-4f65-b5e6-d6394a56da95,{5A62F525-560E-4488-9026-01CA3DC30A6F}:1,{AE8EA766-5078-4ED7-
BC2E-C1A2D580D82C},Connect,200,0,0,2614,5,Unknown,15,1,1979,3,FIRMA\bouska,,,,456b7e4c-ac32-4973-8cf1-2d95f29ee7c5@firma.cz,
cde6d549-a614-48cf-8ae4-89a71a7d7ba8,/o=Firma/ou=First Administrative Group/cn=Recipients/cn=bouska,10.0.1.10,MAIL.FIRMA.LOC
AL,<null>,,MAPIAAAAAOip4Kz8ru+n5sX3x/XF6NjtwPHG5tfm3O3Y4tLmvJ+nkqKSqp6umqKvTgwAAAAAAA==,,{362018D5-977C-4168-8477-95DA73BBEE
3B}:117440123,OUTLOOK.EXE,16.0.10363.20015,2,Negotiate,,,,,,,,,FIRMA\bouska,,Flags=None;,cpn=M_ABR/RUM_ABR/RUM_ABRC/M_APAR/M
_APRH/M_DTC/M_DTQ/M_DTE/M_RDE/M_RDEc/M_DTEc/M_APoRH/M_AER/;cpv=0/0/0/2/2/3/3/3/3/4/4/4/4/;I32:OBudg.C[]=1,MoMTException:2614
 (rpc MaxConnectionsExceeded) -> ;

Resolving the Issue Using Throttling Policy Adjustment for the Organization

Documentation Set-ThrottlingPolicy, Change user throttling settings for all users in your organization

Various connection limits (restrictions on user resource consumption) are set by the Throttling Policy. The default setting is determined by the predefined GlobalThrottlingPolicy. We can list the existing policies using the Exchange Management Shell (EMS).

[PS] C:\>Get-ThrottlingPolicy

Name                             ThrottlingPolicyScope IsServiceAccount
----                             --------------------- ----------------
DefaultThrottlingPolicy_749a82f1 Regular               False
GlobalThrottlingPolicy_30fedd7c- Global                False

We can check if the user has a special policy assigned.

[PS] C:\>Get-Mailbox bouska | FL Name, ThrottlingPolicy

Name             : Bouška Petr
ThrottlingPolicy :

We can check the value of RcaMaxConcurrency, which is our current concern.

[PS] C:\>Get-ThrottlingPolicy GlobalThrottlingPolicy* | FL RcaMaxConcurrency

RcaMaxConcurrency : 40

Policies are created with different ThrottlingPolicyScope, which determines the application.

Global - reserved for the predefined default GlobalThrottlingPolicy
Organization - applies to all users in the organization
Regular - for application to specific users

When we want to adjust the limits for all mailboxes, we will create a new policy that automatically inherits the settings from the default Global policy.

 New-ThrottlingPolicy -Name DefaultMoreConnection -ThrottlingPolicyScope Organization

And then we modify the required value (it is also possible to set it to unlimited)

Set-ThrottlingPolicy DefaultMoreConnection -RcaMaxConcurrency 200

We can also increase some of the other limits

Set-ThrottlingPolicy DefaultMoreConnection -DiscoveryMaxConcurrency 10 -EwsMaxConcurrency 200

Related articles:

Almost since the beginning of my practice, I have been involved in the administration of the Microsoft mail server, i.e. Exchange Server. I started with the 2003 version and worked my way up to Exchange Online. The articles cover many areas of management. Most since the migration to Exchange Server 2016 and its complete configuration. But also Exchange Hybrid and e-mail security.

I was migrating an organization from Exchange Server 2010 to Exchange Server 2016. The whole process was quite demanding and long (it took me 4 months even with my studies). In the process, I came across a number of problems, errors and shortcomings (even in the official documentation). I am creating this series from my notes. This is not a complete guide to the transition, but highlights and mentions of issues I encountered. Individual articles describe different areas of Exchange Server 2016, so it is not only a transition from an older version, but also suitable for a new installation or administration.