Concurrency limits for shared mailboxes

%3CLINGO-SUB%20id%3D%22lingo-sub-2281417%22%20slang%3D%22en-US%22%3EConcurrency%20limits%20for%20shared%20mailboxes%3C%2FLINGO-SUB%3E%3CLINGO-BODY%20id%3D%22lingo-body-2281417%22%20slang%3D%22en-US%22%3E%3CP%3EHi%2C%3C%2FP%3E%3CP%3E%26nbsp%3B%3C%2FP%3E%3CP%3EWe%20are%20considering%20converting%20distribution%20lists%20to%20shared%20mailboxes%20and%20adding%20them%20as%20an%20additional%20account%20(as%20described%20by%20%3CA%20href%3D%22https%3A%2F%2Ftechcommunity.microsoft.com%2Ft5%2Fuser%2Fviewprofilepage%2Fuser-id%2F58%22%20target%3D%22_blank%22%3E%40Vasil%20Michev%3C%2FA%3E%20in%20%3CA%20href%3D%22https%3A%2F%2Fwww.michev.info%2FBlog%2FPost%2F2025%2Fhow-to-add-a-shared-mailbox-as-additional-account-in-outlook-2016-build-8431-and-above-on-windows-10-1709-and-above%22%20target%3D%22_self%22%20rel%3D%22nofollow%20noopener%20noreferrer%22%3Ehttps%3A%2F%2Fwww.michev.info%2FBlog%2FPost%2F2025%2Fhow-to-add-a-shared-mailbox-as-additional-account-in-outlook-2016-build-8431-and-above-on-windows-10-1709-and-above%3C%2FA%3E)%2C%20but%20we%20are%20a%20bit%20worried%20about%20the%20concurrency%20limits.%3C%2FP%3E%3CP%3E%26nbsp%3B%3C%2FP%3E%3CP%3EDoes%20anyone%20know%3A%3C%2FP%3E%3CP%3E%26nbsp%3B%3C%2FP%3E%3COL%3E%3CLI%3EHow%20many%20users%20can%20open%20(connect%20to)%20a%20shared%20mailbox%20simultaneously%3F%3C%2FLI%3E%3CLI%3EHow%20many%20shared%20mailboxes%20can%20be%20opened%20(connected%20to)%20simultaneously%20in%20one%20instance%20of%20Outlook%3F%3C%2FLI%3E%3C%2FOL%3E%3C%2FLINGO-BODY%3E%3CLINGO-LABS%20id%3D%22lingo-labs-2281417%22%20slang%3D%22en-US%22%3E%3CLINGO-LABEL%3EExchange%20Online%3C%2FLINGO-LABEL%3E%3CLINGO-LABEL%3EOffice%20365%3C%2FLINGO-LABEL%3E%3C%2FLINGO-LABS%3E%3CLINGO-SUB%20id%3D%22lingo-sub-2282390%22%20slang%3D%22en-US%22%3ERe%3A%20Concurrency%20limits%20for%20shared%20mailboxes%3C%2FLINGO-SUB%3E%3CLINGO-BODY%20id%3D%22lingo-body-2282390%22%20slang%3D%22en-US%22%3EThank%20you!%3CBR%20%2F%3E%3CBR%20%2F%3EThe%20general%20principle%20is%20well%20understood.%20OWA%20didn't%20come%20to%20mind%20as%20an%20alternative%20-%20thanks%20for%20the%20suggestion!%3CBR%20%2F%3E%3CBR%20%2F%3EWe%20took%20a%20brief%20look%20at%20M365%20Groups%2C%20but%20they%20seem%20to%20stray%20away%20a%20bit%20too%20much%20from%20the%20expected%20workflow%20and%20features%2C%20unfortunately.%3C%2FLINGO-BODY%3E%3CLINGO-SUB%20id%3D%22lingo-sub-2282386%22%20slang%3D%22en-US%22%3ERe%3A%20Concurrency%20limits%20for%20shared%20mailboxes%3C%2FLINGO-SUB%3E%3CLINGO-BODY%20id%3D%22lingo-body-2282386%22%20slang%3D%22en-US%22%3EThank%20you!%3CBR%20%2F%3E%3CBR%20%2F%3EThe%20seeming%20lack%20of%20official%20documentation%20on%20the%20subject%20is%20what%20prompted%20my%20post.%20It%20also%20seems%20I%20have%20missed%20MaxNumExchange%20-%20thanks%20for%20the%20pointer!%3C%2FLINGO-BODY%3E%3CLINGO-SUB%20id%3D%22lingo-sub-2281677%22%20slang%3D%22en-US%22%3ERe%3A%20Concurrency%20limits%20for%20shared%20mailboxes%3C%2FLINGO-SUB%3E%3CLINGO-BODY%20id%3D%22lingo-body-2281677%22%20slang%3D%22en-US%22%3EThe%20less%20you%20have%2C%20the%20better.%20Chances%20are%20not%20many%20people%20will%20need%20to%20have%20the%20shared%20mailbox%20opened%20all%20the%20time%2C%20so%20you%20probably%20want%20to%20avoid%20adding%20them%20for%20everyone.%20Using%20OWA%20is%20also%20an%20option%20or%20even%20preferred%2C%20plus%20it's%20fairly%20easy%20and%20you%20can%20just%20add%20bookmarks%20directly%20to%20the%20shared%20mailbox.%3CBR%20%2F%3E%3CBR%20%2F%3EAnd%20you%20should%20probably%20also%20consider%20M365%20Groups%20as%20a%20replacement.%3C%2FLINGO-BODY%3E%3CLINGO-SUB%20id%3D%22lingo-sub-2281565%22%20slang%3D%22en-US%22%3ERe%3A%20Concurrency%20limits%20for%20shared%20mailboxes%3C%2FLINGO-SUB%3E%3CLINGO-BODY%20id%3D%22lingo-body-2281565%22%20slang%3D%22en-US%22%3EThere%20is%20no%20official%20document%20available%20on%20the%20concurrent%20limit.%3CBR%20%2F%3Eon%20Simultaneously%20open%20a%20shared%20mailbox%2C%20consider%20performance%20issue%20too%2C%20you%20can%20have%20as%20many%20as%20you%20want*%2C%20you%20just%20need%20to%20tweak%20the%20registry%20on%20the%20device%3CBR%20%2F%3E%3CBR%20%2F%3EHKEY_CURRENT_USER%5CSoftware%5CPolicies%5CMicrosoft%5CExchange%3CBR%20%2F%3EDWORD%3A%20MaxNumExchange%3CBR%20%2F%3EOutlook%202010%20values%3A%20a%20decimal%20value%20between%201%20and%2015%3CBR%20%2F%3EOutlook%202013%20values%3A%20a%20decimal%20value%20between%201%20and%209999%3CBR%20%2F%3EOutlook%202016%20values%3A%20a%20decimal%20value%20between%201%20and%209999%3CBR%20%2F%3EOutlook%202019%20values%3A%20a%20decimal%20value%20between%201%20and%209999%3CBR%20%2F%3E%3CBR%20%2F%3E%3C%2FLINGO-BODY%3E
New Contributor

Hi,

 

We are considering converting distribution lists to shared mailboxes and adding them as an additional account (as described by @Vasil Michev in https://www.michev.info/Blog/Post/2025/how-to-add-a-shared-mailbox-as-additional-account-in-outlook-...), but we are a bit worried about the concurrency limits.

 

Does anyone know:

 

  1. How many users can open (connect to) a shared mailbox simultaneously?
  2. How many shared mailboxes can be opened (connected to) simultaneously in one instance of Outlook?
4 Replies
There is no official document available on the concurrent limit.
on Simultaneously open a shared mailbox, consider performance issue too, you can have as many as you want*, you just need to tweak the registry on the device

HKEY_CURRENT_USER\Software\Policies\Microsoft\Exchange
DWORD: MaxNumExchange
Outlook 2010 values: a decimal value between 1 and 15
Outlook 2013 values: a decimal value between 1 and 9999
Outlook 2016 values: a decimal value between 1 and 9999
Outlook 2019 values: a decimal value between 1 and 9999

The less you have, the better. Chances are not many people will need to have the shared mailbox opened all the time, so you probably want to avoid adding them for everyone. Using OWA is also an option or even preferred, plus it's fairly easy and you can just add bookmarks directly to the shared mailbox.

And you should probably also consider M365 Groups as a replacement.
Thank you!

The seeming lack of official documentation on the subject is what prompted my post. It also seems I have missed MaxNumExchange - thanks for the pointer!
Thank you!

The general principle is well understood. OWA didn't come to mind as an alternative - thanks for the suggestion!

We took a brief look at M365 Groups, but they seem to stray away a bit too much from the expected workflow and features, unfortunately.