Why Messages in Your Exchange Online Inbox Are So Large

Sixty Times Larger Than in 1996

Browsing the pages of my 1996 book on Microsoft Exchange Server 4.0, I discovered that the average size of an email 25 years ago was in the 2-4 KB range. This got me thinking about what the current average might be, so I checked my Inbox by running the Get-ExoMailboxFolderStatistics cmdlet to retrieve the total size of the folder and the number of items in it. The code I used is as follows:

$CheckName = Read-Host "What User Mailbox Shall I Check?"
If (Get-ExoMailbox -Identity $CheckName) {
   $Stats = $Null
   Write-Host "Checking Mailbox for" $CheckName...
   $Stats = Get-EXOMailboxFolderStatistics -Identity $CheckName -FolderScope Inbox | Select Name, ItemsInFolder, FolderSize ,@{Name="FolderSizeBytes"; expression={((($_.FolderSize -replace '^(.*\()(.*)(\sbytes\))$','$2').Replace(',','')) -as [int])}} 

    Write-Host ("Results for mailbox {0}: {1} Inbox items {2} folder size with an average message size of {3} KB" -f $CheckName, $Stats.ItemsInFolder, $Stats.FolderSize.Split("(")[0], [math]::round(($Stats.FolderSizeBytes/$Stats.ItemsInFolder)/1KB,2))}

The code looks complicated, but it’s not too bad. We fetch some of the properties returned by Get-ExoMailboxFolderStatistics and put them into a variable. One of the properties is calculated to return the total number of bytes in the folder as a integer variable. We then calculate the average size of a message and write out what we found.

In any case, the result is that the average size of messages in my Inbox is sixty times larger than they were in 1996. Other mailboxes will likely show different results (run the code against your Inbox to see what it reports) as it all depends on the volume of traffic and the type of messages you receive. However, I think everyone can agree that the size of messages sent and received today are larger than they were in the past.

Reasons for Swelling Messages

The reasons why today’s messages are much larger vary across organizations. However, I think four core reasons exist:

  • More Headers: Today’s SMTP traffic include more message headers than ever before.
  • Rich Editors: Old text-only messages are now jazzy HTML rich-text messages.
  • Fat Attachments: Capable networks make users think nothing about sending a 25 MB PowerPoint presentation in email. They wouldn’t do this if connected on a dial-up telephone link.
  • User Habits: We’re lazy and reply-all to messages which include all previous replies.

SMTP messages used to include just enough headers to get the email through. Now, a bunch of new headers exist, some of which are essential to authenticate that email comes from the right source and isn’t spam (DMARC, ARC, SPF, etc.). Other x-headers are added by servers like Exchange to track the progress of messages, include anti-spam reports, and measure latency. Fire up Outlook’s Message Header Analyzer add-in to examine the headers in a message to see what I mean.

Rich Format Email

Rich editors mean that text and formatting instructions must be sent where plain text was the norm. Embedded graphics drive up message sizes, including the autosignatures inserted by users and organizations. Complex autosignatures with corporate logos and social networking links add even more to message sizes, especially when organizations lose control and insist that every message goes out complete with graphic-intense marketing messages.

It’s lucky for everyone that servers are so fast and networks so capable that they can deal with the processing overhead required to transport messages in and out of Exchange Online and between Microsoft’s datacenters and email servers elsewhere across the internet. I’m not sure that Exchange’s original IMC (Internet Mail Connector) would have been up to the task.

The Size and Intelligence of Notification Email

Even relatively simple notification messages generated by applications can be huge. For instance, a brief review of notifications from different Microsoft 365 apps telling me that people are trying to contact me resulted in the following sizes:

  • LinkedIn: 174 KB
  • Teams: 265 KB,
  • Yammer: 472 KB (Figure 1).
Yammer's intelligent missed conversation message is > 400 KB
Figure 1: Yammer’s intelligent missed conversation message is > 400 KB

To be fair to Yammer, the latest generation of notification email are graphically rich and interactive to allow recipients to respond within the email client. The code necessary to enable this functionality increases the amount of HTML carried around in messages. It’s a price we must pay for intelligent messages.

Cloudy Attachments and Bad Habits

Microsoft’s efforts to convince users of the goodness of sending “cloudy attachments” (links to files in OneDrive for Business and SharePoint Online) are showing signs of progress, especially within Office 365. Consistent sharing links, the ability to co-author (now even when documents are protected with sensitivity labels), and easy access to files stored in other tenants mean that users send an increasing number of links where files were once attached. However, the default option for many is still to attach files (or rather, a copy of a file), and once that happens, the average size of message swells quickly.

People are creatures of habit, which is one reason why we continue to send traditional attachments. We also tend to take the path of least resistance, which is why we reply to messages without thinking about the long trail of previous replies sent along with our response. The outcome is that a message at the end of a thread might be 1 MB, roughly 10% of which is useful non-repetitive information.

Large Cloud Mailboxes Required

The world of email is very different to what it was when Exchange 4.0 appeared in 1996. I doubt anyone today could cope with the 20 MB mailbox quota often assigned to users in those days. Given the growing size of messages, it’s a good thing that Microsoft makes generous mailbox quotas available for Exchange Online users. Even so, if the trend of ever-swelling messages continues, we’ll all need 200 GB mailboxes soon.

The Office 365 for IT Pros eBook hasn’t covered developments in Exchange mailbox technology since 1996. Some of our authors remember back that far, but mostly we live in the modern world and track new developments as they happen inside Office 365.

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