Typical Limits

Way back in Chapter 1, "Introducing Exchange Server 2007," we talked some about the changing nature of e-mail and how message sizes and mailbox content has changed. Later, in Chapter 3, "Designing a New Exchange 2007 System," we included a section called "Planning Your Disk Space Requirements." In that section, we took you through a planning exercise to figure out roughly how much disk space you will require given maximum mailbox sizes.

Note 

Regardless of what we may tell you are typical limits, you must find the right balance for your own organization. That balance may be far above or below the limits we recommend.

If your organization has not set mailbox limits or message size limits, then your first challenge is to figure out what those actually need to be. If you currently have limits in place, then you need to determine if they are effective. Let's first look at mailbox storage limits. A question we are frequently asked is, "What is typical for the industry?"

Unfortunately, there is no set guideline for what "the industry" is doing with respect to mailbox limits. In the Exchange 2003 time frame, these limits are commonplace:

Issue warning	250MB
Prohibit send	275MB
Prohibit send and receive	300MB
Maximum outgoing message size	10MB (the default)
Maximum incoming message size	10MB (the default)

We have had individuals in some large corporations and European organizations report to us that the following limits are more commonplace. Notice that the message sizes are higher, but the mailbox limits are a bit lower:

Issue warning	150MB
Prohibit send	200MB
Prohibit send and receive	300MB
Maximum outgoing message size	20MB
Maximum incoming message size	20MB

Some organizations look at these limits and wish they could provide their users with as much mailbox space, while other organizations see these limits as clearly insufficient for their messaging needs. Exchange 2007 has not yet been in use long enough to see any clearly defined trends, but here are limits that we are using for planning capacity for a typical organization:

Issue warning	500MB
Prohibit send	550MB
Prohibit send and receive	600MB
Maximum outgoing message size	10MB
Maximum incoming message size	10MB

These limits have to be carefully balanced with available disk space, backup and restore resources, and users' needs.

Tip 

If your users require lots of mailbox space for official business reasons, you should find a way to support that requirement.

As we mentioned in an earlier chapter, some people at Microsoft are already hinting that a 2GB mailbox limit will become commonplace. Indeed, the default Exchange 2007 mailbox database limits are as follows:

Issue warning	1,945MB
Prohibit send	2,048MB
Prohibit send and receive	2,355MB

While most of us would personally love to have a 2GB mailbox on our corporate mail server, there are a number of factors that you should consider when setting limits.

Tip 

The default mailbox storage limits for mailbox databases is now 2GB. Please keep this in mind when creating a mailbox database. This may be far higher than you want for your users.

Once you have evaluated some of these factors and decided if you need to raise your limits, you need to find a way to effectively increase your technological capacity to support these limits. This includes larger disks, faster backup/restore capabilities, and larger backup/restore capabilities, and it could include additional technologies such as archival solutions. The following list includes some of the factors you need to consider:

• Is there a business reason for giving users more mail storage? Consider the cost of a user trying to find information that at one time was in their mailbox and is now gone.

• Evaluate your current limits (if you have them). Do you users have to delete/archive mail frequently in order to stay under their prescribed limit? Do they require e-mail that they have deleted or archived?

• Consider your VIPs, executives, and other heavy messaging users: Do they need more or less space than a typical user? A heavy messaging user may not necessarily be a VIP. In some organizations, the salespeople, engineers, architects, or paralegals may be the heaviest users of messaging technologies.

• Do you have an e-mail archival system in place that automatically moves older mail out of a user's folders or managed folders and stores that data in a long-term storage repository? Centralized repositories may actually reduce mailbox space requirements since data can be retained for much longer periods of time without actually impacting the mailbox database. However, retention periods for the content in a user's mailbox will still depend on allowing the user to access that data via Outlook, Outlook Web Access, Outlook using RPC over HTTP, or Windows Mobile devices. Depending on the archival system, older messages might not be accessible from all devices or locations, so this should be considered when planning for retention times.

• Basic mailbox content includes e-mail messages, appointments, contacts, tasks, and journaling information. What other technologies are you employing that would increase the average size of a mailbox?

  • Integrated faxing? How many faxes would your users need to retain?

  • Integrated voicemail? How many voicemail messages would your users need to retain?

  • Forms routing and workflow? Would a historical record of forms processed (expense reports, purchase requests, travel approval, etc.) need to be retained by the sender as well as other people in the forms routing process?

• Are you faced with regulatory requirements that require the retention of certain types of information for a long period of time?

• Could larger mailbox sizes expose you to potential denial-of-service attacks where someone sends lots of large messages to your users until all mailboxes and possibly the database or transaction log disk are filled?

These are just some of the factors that may affect your decisions with respect to assigning mailbox limits. Just when you think you are out of the woods with respect to this decision, you have another decision to make. What about message size limits?

The default message size limit for both incoming and outgoing messages is 10MB; this limit has not changed since Exchange 2003. This value is not just an arbitrary or nice round number; it is based on Microsoft's best practices and feedback from many customers. We often find this limit to be insufficient in many environments where large attachments are used. Regardless of whether or not it is a good practice to send a 50MB file through e-mail, sometimes it is just not possible for users to easily transfer large attachments between organizations. If your organization requires the transfer of large files, always try to find an alterative such as using portals, websites, FTP servers, or instant messaging.

Note 

Just because you raise your organization's outgoing message size limit does not necessarily mean that a remote organization will be able receive messages as large as your users can send.

So what are some factors to take in to consideration when planning for a maximum message size? This is not as clear-cut and easy to handle as mailbox size limits, but here are some things to think about:

• Do your users need to transmit large file attachments?

• Do you have a mechanism to prevent "large attachment" abuse such as preventing MPG, WMA, WMV, and MP3 files from being sent via e-mail, if those file types are considered inappropriate in your organization?

• Is there an alternative to sending or receiving large attachments, such as allowing internal users to post content to a SharePoint portal page; using extranet pages, intranet solutions, or instant messaging; or providing external users with an alternative way to send you large files?

• Can large content be automatically compressed by some client-side utility, thus reducing attachment sizes?

• If large message sizes are allowed, can your network links and Internet connection handle the capacity?