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Outlook resource problems: a difficult diagnostic dilemma

1. Background

If you are encountering Outlook problems or supporting someone who has such, look at our Outlook support checklist for a comprehensive troubleshooting guide.

For more information see ourOutlook support checklist (pdf).

You may experience colleagues with issues regarding "automapping", "too many folder" and 'synch issue" problems with Nexus. Occurrences of these problems are difficult to diagnose as:

  • The problems tend to be intermittent (and poorly defined for the Nexus Team to investigate)
  • The cause of the problem (we suspect) can be multi-dimensional. As well as general software malfunction issues, there can be a combination of:
    • Too many folders
    • Too many connections (multiple clients all connecting at the same time)
    • Too many mailboxes mapped into Outlook (automapped or otherwise)

2. Software affected

Microsoft Outlook (various versions, possibly including Outlook 2011 for Mac).

3. Problems you may have seen


  • People using Outlook with Full Access to one or more other mailboxes have a problem with folders not being up to date as regards content.
  • In extreme cases performance problems with Outlook may be seen: possibly with the client ‘locking up’ for a time or crashing
  • Entries in the synchronization logs that include "Your server administrator has limited the number of items you can open simultaneously".

4. What does the ‘500 folders per Exchange mailbox server limit’ mean?

The Nexus Exchange service has ten mailbox servers (at the time of writing). Each Nexus mailbox resides on one server, although database availability groups and failover means that this is not limited to one piece of physical hardware. Exchange has a default limit of 500 folders per mailbox server when accessed via Microsoft Outlook.

5. What is automapping?

Automapping is a labour-saving device that was introduced with Exchange 2010. Previously, if you had Full Access to another mailbox (another person’s or a generic/project account), you – the user – had to map in that mailbox to your copy of Outlook for you to be able to access it. This involved several clicks and was considered to be ‘rather technical’. Now, if you are given Full Access to another mailbox, Exchange sets that mailbox to ‘automap’ in Outlook and it appears automatically without having to click on or type anything.

Possible related problems include:

  1. If you had already manually mapped that mailbox in, you may get two entries, both of which cache (N.B. this only occurs when an individual is given Full Access anew. If Full Access was granted in the more distant past this problem should not be apparent.)
  2. The two entries (from the situation above) can confuse each other as to where to file Sent Items (see
  3. If you are particularly unlucky the second mailbox may push you over the ‘500 folders per Exchange mailbox server limit’.

For more information about when automapping is useful and when you may wish to have it switched off, see the following:

6. What do you mean by ‘too many connections’?

This problem is not limited to Outlook and IMAP clients can have similar issues. Multiple machines/devices connecting to the same mailbox(es) can cause or exacerbate performance problems. Whilst some session limits are per IP address, many are per authenticated session so if (for example) your mailbox contains 251 folders and works quite happily on one machine, and if you leave Outlook running at home, and possibly also on your terminal server, and possibly also on your mobile device you may easily exceed your available system resources and you will see problems.

7. Is the 500 folder limit a major problem?

For almost all users, 500 is an awful lot of folders and you have to be quite unlucky to have your personal mailbox and a secondary mailbox both being on the same mailbox server (currently, a one in ten chance).

However, it has been coming to light that there are a few people across Nexus with many hundreds of folders in their personal/primary mailbox and they then have Full Access to many secondary/project mailboxes. For them the chance of running into the folder limit is significantly higher. These people will possibly see caching problems and ‘sync errors’.

A few of these people are also benefiting from automapped secondary mailboxes, and a few of these people may have already (manually) mapped the secondary mailbox into Outlook. This doubles up their problem, although it should be quickly alleviated by deleting the manual mapping.

8. What can you/we do?

Some have suggested setting the secondary mailbox connection to be uncached. This can be a useful temporary work-around, but can lead to further problems, so is not a good state to be in long-term.

The Nexus Team can turn off automapping for a user-secondary mailbox relationship, if requested. However, if the folder count problem is very serious, this may merely improve the situation rather than cure it entirely. Also, this may only benefit a very few users with the ‘too many folders’ problem. If you do want automapping disabled for some of your users’ mailboxes, please open a ticket with the IT Services Help Desk detailing each user-secondary mailbox pair.

9. Generally...

We suggest...

  • Vigilance in keeping numbers of folders down. With 'modern' search techniques great trees of folders categorising content are really not needed. If someone's 'archive' folder structure could be flattened, this will help enormously.
  • Interrogate users as to their other 'devices'. Are they connecting many times, possibly from many places? It is quite normal for people to forget a few 'places'.
  • Check whether the same mailbox is mapped into Outlook multiple times.


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Written by IT Services. Latest revision 18 October 2017