Users opening multiple shared mailboxes results in poor performance in Outlook Desktop


I'm looking for a new solution to a problem for a new client in terms of a business process they follow. They're bookkeepers and have around 10 staff members opening up additional shared mailboxes to process client requests. 



Platform - Office 365 and Exchange Online

Desktop Application - Outlook

Number of users - 10

Workflow - If any of the team members receives an email from a client and it is considered a processable request they move this email to a shared mailbox called Shared 2016 (they have one for each year). In that mailbox there is a To Do folder as well as a dedicated folder for each of their clients. Once the request has been completed the email gets moved to the client's dedicated folder. 

Typical Outlook Setup: Any of these users at any point will have around 7 mailboxes open in Outlook. Their own mailbox as well as 6 shared mailboxes. 


Note: The combined space for the 6 shared mailboxes is 22Gb


The Problem

They started off with Exchange on premise and this worked fine, but since then they have moved to Exchange Online and that's where the process took a nose dive. 

Assessing the situation is difficult as I'm seeing a few different results, but the main painpoint are: 

  1. Occassionally when a user moves mail from their own mailbox to the shared mailbox it doesn't show up for everyone
  2. In the shared mailbox, when they try to open an item, they're frequently getting error messages such as "Could not open item. Try again". 
  3. When they try to move items from their own mailbox into the shared mailbox they frequently get the same errors as in point 2 above
  4. Outlook performance is degraded dramatically from what we're used to seeing with other customers. 
  5. We've attempted this using Cached Exchange Mode and Connected Mode
    1. Cached Mode we see more of the problems found in point 1
    2. In either mode we see more of the problems described in points 2-4
  6. Another odd problem we're seeing is that if a user starts typing an email and doesn't send it quite quickly they get the message "The function cannot be performed because the message has been changed".


What's the Solution

I can't think that having 7 mailboxes open is healthy but I'm also not entirely convinced this is the problem. 


Question: Is the there a way to troubleshoot performance issues as well as the errors we're discovering. 


What I am wondering is if an entirely different approach isn't the best answer: 

  1. Use SharePoint sites with site mailboxes
  2. Use Exchange Public folders

I don't know if Exchange Public folders will handle this any better. 


Anybody out there with a similar configuration... I'd appreciate hearing about it. 

3 Replies

7 mailboxes and 22gb of data in a single Outlook profile is a bit too much and is what's most likely causing the issues you are describing. Ideally, you would aim to limit the number of mailboxes accessed in the same time, by configuring them in seperate Outlook profiles or using OWA. Limiting the amout of data synced by controling the cache is another options, you can do this if you configure the shared mailboxes as additional accounts in Outlook (add them via File -> Add account -> enter the shared mailbox address -> enter your credentials when prompted).

Thanks for the response Vasil. 


Is there a definitive way to establish that Outlook is having trouble with this configuration. I mean I personally agree and suspected this wasn't a good setup, but the customer insists that when they were using on-premise they never had these issues. There's obvious reasons why they may have had better performance but I'd like to be able to nail down the exact reason why Exchange Online is not going to offer them the same performance. 


I just tested SharePoint site mailboxes but the performance from Outlook 2016 client would make this a bad choice. It's handy in that you see the mailbox in Outlook client as well as the documents library, but ironically you can't move files around in the Documents folder from Outlook. I've read elsewhere that you should also see the site mailbox in OWA but so far I don't see it. So I'll have to investigate that further as well. Also, in Outlook client if you right click a document in the Documents Folder and click forward you get the error "The operation failed. The messaging interfaces have returned an unknown error. ...". You can't send on behalf of the site mailbox either and when you create a new email, albeit from your own address, you can attach anything from the SharePoint site's document library, so I don't see the usefullness of the documents showing in Outlook. 


Feels to me like so many of the collaborative features MS is putting into the Office 365 platform simply don't work or require more configuration to make them work and sometimes that doesn't even help. 


Do you have a link where I can see how to configure or control the cache as you suggested? 


Site mailboxes are deprecated, you should look into Modern groups instead. Though they are not exactly a replacement for shared mailboxes, and if they are used to rules or moving messages, sticking with shared mailboxes is the way to go.


The cache settings will be exactly the same as with any other mailbox you add in Outlook, the idea here is that you will be opening the shared mailbox "directly", giving you more control over the functionalities exposed in Outlook. Here's a short guide I did a while back, in case you need more details: