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Three approaches to setting Exchange Server mailbox size limits

The maximum mailbox size administrators should allow on Exchange Server is always a subject of controversy. How big should they be? Or, perhaps it would be more appropriate to recast the question as, how small should they be?

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Because there's never any one hard, definitive answer to this question, many Exchange Server administrators are tempted to simply set an arbitrary mailbox size and be done with it. This is never a good idea. For one, future growth can mean the mailbox sizes you set will be quickly exceeded, so you'll just have to redo them.

There are two basic ways to purposefully set mailbox limits: flat per-user limits and tiered limits. Additionally, you could analyze real-time usage and set customized mailbox quotas.

Flat per-user limits

A flat per-user limit is easy enough to calculate: take the total size of the available Exchange Server message store space and divide that by the total number of users in your organization. This will yield a value you can use as a starting point for how big your Exchange Server mailboxes should be.

If you are using Exchange 2003 Standard Edition (with a maximum single-database size of 75 GB) and you have 30 users, 75 divided by 30 equals 2.5. That is, you could devote up to 2.5 GB of mailbox space per user. It's not likely that every Exchange Server user is going to accumulate that much email, but at least it creates a limit where before there was none.

There may be some other ways to divide up the allocation. Daniel Petri has a good article on this subject, Calculating storage space in Exchange 2003, which goes into some detail about planning for future growth needs (i.e., how much space to allocate for the Exchange Server message store to grow into over time).

Tiered limits

Tiered limits involve splitting up users into groups and determining what the total usage for each group will be. If you have 30 Exchange Server users, with five of them being department heads that send and receive much more email than the other 25 regular employees, you could split things up in a tiered fashion.

For instance, you could give each tier half the store (about 38 GB if we're continuing the above example), and divide each portion of the store equally. This way, the regular users get 1.8 GB or so, and the department heads get 7.5 GB or so each; the allocation will be proportionate to their usage and needs.

Analyze real-time usage

More on Exchange Server mailbox management:
When Exchange Server mailbox size quotas break

Set size limits for all local email messages

Create custom mailbox quota messages for Exchange 2003

Setting age and size limits on users' Deleted Items folders

Managing Exchange recipients and distribution lists

Exchange Server mailbox management tips and resources

A third way to handle email allocation is by analyzing real-time usage. See how much email a user is actually using, consult with them, and set a limit based on that. Consulting with the user may only be practical in a smaller organization though; most of the time, the admin will probably end up working with usage statistics.

It's relatively easy to get a list of the available mailbox sizes, but for more detailed reporting a third-party product like Quest MessageStats might be more in order.

The one thing you should never do is simply pick a number out of a hat that doesn't reflect your actual usage or needs, even provisionally. Take the time and effort to find out how much mailbox space is really required.

About the author: Serdar Yegulalp is editor of Windows Insight, a newsletter devoted to hints, tips, tricks, news and goodies for all flavors of Windows users.

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You state in your article that the maximum single database limit for Exchange Server 2003 Standard Edition is 75 GB, but I believe it is 16 GB.
—Steve M.

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As of Exchange 2003 SP2, the maximum database size limit was raised to 75 GB. It does default to a maximum of 18 GB out of the box, but can be raised to 75 GB with minimal effort (i.e., a registry entry).
—Serdar Yegulalp, tip author

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You don't mention how to implement tiered mailbox quotas. Would the mailbox quotas need to be manually set per user or can I link the quota tier to an Active Directory group membership? If so, how can I do this? I can't seem to find any information on it.
—Justin F.

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Microsoft spells out the quick-and-dirty way on how to set mailbox quotas on Exchange storage groups, and Microsoft Exchange developer Glen Scales has some posts on his blog on how to script mailbox quotas using PowerShell.
—Serdar Yegulalp, tip author

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This was first published in January 2007

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