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Exchange Mailbox Limits Update Lag

March 19, 2009

Microsoft Exchange is a fairly ubiquitous system for handling e-mails and I made a surprising discovery today whilst amending a user’s mailbox storage limits … they don’t get updated straight away.

So lets take a little step back and look at storage limits.  First of all Exchange allows three size limits (in KB not number of mails) to be placed on user mailboxes. The first level triggers a warning to the user that they are approaching a limit … ironically the warning is an e-mail but it is a small one so this can be forgiven.  The second restricts the user’s permission to send mail (but they can still receive it).  The third limit actually stops any mail entering the inbox, but the mail server will keep mails queued up for a little while to give the user chance to make some space so all is not immediately lost if the user reaches this third and final limit – which can happen if someone bombards you with lots of e-mails with large attachments.

We impose limits on mailboxes for a number of reasons.  Ones that immediately come to mind for example are (in no particular order):

  1. It ensures that we have enough storage capacity on the mail server to handle all of the mailboxes.
  2. It ensures that users can’t monopolise the available storage space.
  3. It ensures that a user’s mail software remains responsive (though PST files can *really* trash this performance I also noted today).
  4. It ensures that mailboxes can be backed up and restored efficiently.

Occasionally however people have a need to have an increased mailbox size to get them through a busy period where they may be out of the office and unable to spend time housekeeping or if they are receiving a number of large attachments (e.g. during a funding bidding process or event registration).  On these occasions a temporary increase in mailbox size is authorised for an agreed period.

This isn’t a common occurrence and I’ve probably only amended mail storage limits some half a dozen times or so.  This change has always been picked up within a few minutes and until today I hadn’t realised how ‘lucky’ I’d been.

Today I discovered that Exchange actually works from a cached set of mailbox limits and that the default turn around time on this is up to a whopping two hours.  So this is something we’ll need to keep in mind in future though it may be that we might consider applying a registry change at some point to remedy this (though I think this is one to test on our development environment before making the change to the live system) or at least identify roughly at what time the cache refresh runs so that we can give our users a better idea of when the limits will take effect.

Whilst the #326252 Microsoft Knowledge Base article applies to Exchange 2000, some background reading suggests that this is applicable to later versions of Exchange too.

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