Windows Explored

Everyday Windows Desktop Support, Advanced Troubleshooting & Other OS Tidbits

Sizing It Up – How Encoding Impacts Successful Message Delivery

Posted by William Diaz on August 1, 2011


Because of the way e-mail messages are encoded, several more MBs can be added to the size of a message, causing it to exceed the message send restrictions defined on the Exchange server1. As a result, there can be some confusion, since Outlook will show that the message is well below the size restrictions. For example,  we cap our message size restrictions at 24MBs. However, we might have a recipient that never receives a 20MB message or is not able to send a message of the same size.

Lets take the example here where I have created a message and attached two Word documents, 10MBs and 11MBs respectively:

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Shortly after sending, I receive an undeliverable message:
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Outlook shows the message size is 20MBs total:
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I should have 4 MBs of breathing room. What’s happening? I can get the details from our email gateway:
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The message was rejected and the Final Rule states a this is due to maxsize, which is 28901937 bytes, about 27MB+. But how did we go from 20Mbs to 27Mbs? In short, this can be attributed to message encoding. Take base64, for example: this algorithm “is based heavily on the Privacy-Enhanced Mail (PEM) standard that is defined in RFC 1421. Base64 encoding uses the 64-character alphabet encoding algorithm and output padding characters that are defined by PEM to encode the message body data. A Base64 encoded message is typically 33 percent larger than the original message. Base64 encoding creates a predictable increase in message size and is optimal for binary data and non-US-ASCII text.

This is not to say that every message will be 33% bigger during encoding. Some attachment types are handled better during encoding than others, so a message with a zip file or PDFs may not grow as big as a message with several Word documents. This also does not mean that, for example, the person that receives the message will see a larger attachment. Message encoding is, for the most part, transparent and usually the attachment received is the same size as it was sent. However, there are cases where sometimes the message attachment indicates it is larger than the size it is when it was sent in the email client software, only to appear on disk as the actual sent size.


1 Firewalls and other security software can also define message size policies that affect the delivery of emails, not just Exchange rules.

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