Enterprise Message Size Limits



In the ongoing administrator versus user fights, message size limits always seems to come up. Why? Users feel they need to attach that PowerPoint slide deck with movies and sound that is well over 100MB and send it to everyone in the organization. Technically, yes, Exchange and Outlook can send this. Realistically, this is a really, really bad idea because it will bog down the server while it sends and transaction logs will fill up. So, we implement size limits on the sending of e-mails to prevent users from harming themselves and the server. Unfortunately, this requirement should come from the business unit to the IT department to tell IT how large a message is acceptable. Normally, IT departments decide on what their systems can support and choose a number there. I have seen organizations with 1MB size limits and some up to 30MB. As storage and bandwidth increase, message sizes follow suit. The big question you need to ask is can your organization survive someone accidently sending that 30MB PowerPoint attachment to everyone in the GAL? Plan for the worst and hope for the best.

The concept behind message size limits works in a "reverse funnel" process. The idea is that messages entering the environment that do not meet the size requirements get filtered/rejected first. Typically, you would want the smallest message size in the perimeter of the network, on an Edge server or appliance. That way, the message over the size limit never reaches the Exchange environment where it starts to become more costly to block. This minimizes the unnecessary processing of messages. Now, internally is probably a different story as internal traffic is probably cheaper. Here, message limits need to go on the Hub servers to intercept messages in transit. Different departments can have different limits, such as an art department that send large graphics to one another. Of course, e-mail is not the best way to share documents (that is what SharePoint is for) but we digress. For departments, you will want to configure this on the user level because this is the first step checked in the message size limit process. User limits can override organizational controls, so be careful when setting these.

In Exchange 2010, the default message size is 10MB with a recipient limit of 5,000. The total message size is what matters, and this includes the header, attachments, and content of the message. Exchange can filter by attachment size as well, but more organizations filter by total size. Recipient limits count as unique entries in the TO, CC, or BCC field, so that a distribution list would count as a single recipient. The good news is that Outlook will check for restrictions before sending, so the user will receive an over the limit message before the mail message enters the Exchange environment. This is a good thing because in the early days, a user could attach a 1GB file, send it, wait for it to process, THEN Exchange would send a message saying it was over the limit.

Understanding Message Size Limits

?Setting Mailboxes and Public Folders

Working from the mailbox out, this is how to configure size limits for an organization.

1. For mailboxes, perform the following steps:

a. In the console tree, navigate to Recipient Configuration > Mailbox.

b. In the result pane, select the mailbox for which you want to configure message size limits.

c. In the action pane, under the mailbox name, click Properties.

d. In <Mailbox User> Properties, click the Mail Flow Settings tab.

e. Select Message Size Restrictions, and then click Properties.

f. Proceed to Step 3.

2. For mail-enabled public folders, perform the following steps:

a. In the console tree, click Toolbox.

b. In the result pane, click Public Folder Management Console, and then in the action pane, click Open Tool. The Public Folder Management Console appears.

c. In the console tree, expand Default Public Folders, and then click the public folder that you want to configure. If the public folder you want to configure is a top-level public folder, click Default Public Folders.

d. In the result pane, select the public folder for which you want to configure message size limits.

e. In the action pane, under the public folder name, click Properties.

f. In <Public Folder> Properties, click the Mail Flow Settings tab.

g. Select Message Size Restrictions, and then click Properties.

h. Proceed to Step 3.

3. In the Message Size Restrictions dialog box, select the Maximum message size (in KB) check boxes to set the maximum size for messages that can be sent and received by the mailbox or public folder. Use the corresponding text boxes to type the maximum message size allowed in kilobytes (KB). The message size must be from 0 through 2,097,151 KB. If a message larger than the specified size is sent to the mailbox or public folder, the message will be returned to the sender with a descriptive error message. Click OK to return to the Mail Flow Settings tab.

4. Click OK.

Configure Message Size Limits for Mailboxes or Mail-Enabled Public Folders

Setting the Transport

Next step is to configure the Hub transport settings. Fairly easily, these are configurable through the EMC or EMS, depending on whichever you are more comfortable with. This works the same on an Edge server, but if your organization uses something different, then get with the product vendor to determine the correct way to configure the device.

Included in the content below is information about global settings. These affect all the hub transport servers in their configuration. While you cannot change the message size limits per hub server, you can change some aspects such as maximum recipients per message.  

Configure Transport Settings Properties


Outside of Outlook

Message limits come into play in two more areas that lots of companies forget about. Entourage, or Outlook for Mac, uses EWS in Exchange 2010 to send messages. While the users are still affected by the organizational message limits, they have a separate configuration necessary to allow them to send large messages, or anything over the default 10MB. This article is for Exchange 2007, but it is applicable to Exchange 2010. Please note: Entourage HAS to be running the latest version that uses EWS as previous versions run WebDav, which is no longer available in Exchange 2010. This is a free upgrade.

Contact Microsoft for Entourage Upgrade

Allow Entourage to Send Large Attachments

Also, OWA has a separate entry for sending larger than 10MB attachments.

Configure Maximum Message Size in OWA

Finally, should you send a 9MB message in a 10MB environment and receive an error that your message is over the limit, blame message conversion. Essentially, Outlook runs MAPI whereas the Internet runs MIME, so there are conversion penalties for this. It used to be around 30%, as in, your 9MB message is really 12MB, but it has come down in Exchange 2010.

Frequently Asked Questions About MIME and Content Conversion

Recipient Restrictions

Beyond the size of a message, an organization can configure other limits, such as limits on who can send to whom. For example, an organization may want to limit who can send to “All Mail Recipients” as it contains 90,000 mailboxes. Administrators can apply this to distribution lists as well as to mailboxes.

Understanding Recipient Restrictions


The Exchange team set the message size limit at 10MB for good reason. Most companies running Exchange 2010 have solid and robust Internet connections as well as the hardware needed to run large messages. The question becomes why should a company increase these limits? Earlier, the purpose of e-mail came up, as in, should a company even use it to send documents. There are a lot of other systems available that are optimized to handle large document stores, such as SharePoint. Exchange does not function well as a document repository, but it does function. Sending a link to a document in a SharePoint site takes up much less space, provides for more accountability, and is overall cheaper than maintaining large Exchange databases.

However, many companies do not use SharePoint or a similar product. Thus, it comes down to Exchange to offer this solution. If a user can send a 20MB attachment, there will be 20MB attachments floating around the Exchange environment. Look at what the organization does and what messages they send to each other. Art departments or engineering firms that send CAD drawings may have solid arguments to raise the limits, but PowerPoint slide decks or simple photos may not. Some tracking log analysis will detail what is actually sent over the environment and help establish the smallest message size possible to fit the organization.