email size with attachement increased in size

http://technet.microsoft.com/en-us/magazine/2009.01.exchangeqa.aspx?pr=blog

Exchange Queue & A Mysterious Attachment Size Increases

Q: Our organization's messaging infrastructure is based on Exchange Server 2007. We have a relatively strict message-size limit of 12MB set throughout the organization.

We have observed a strange behavior that seems to be related to the size of files attached to a message. When sending an e-mail message to an external user with, let's say, an 11MB attachment, the message is delivered to the recipient as expected. But if this message (including the attachment) is forwarded back to the sender on the internal network, the sender gets a non-delivery report (NDR), indicating that the message is larger than the current system limit or that the recipient's mailbox is full.

After taking a close look at the issue, we can see that at some point after the message leaves the organization, the size of the attachment increases by approximately 30%. The question is, why do attachment sizes increase while sending and receiving e-mail messages through the Internet? And more important, is this expected behavior?

A The short answer is yes. This is often expected behavior, not only for Exchange Server 2007 but also for earlier versions of Exchange Server as well as any other messaging system that supports MIME (Multipurpose Internet Mail Extensions) and uses Base64 to encode attachments. When an internal Exchange user sends a message to a recipient inside the Exchange organization, the message doesn't require any content conversion. This means that you won't see the message or attachments increase in size when they are delivered. Messages sent to external recipients, on the other hand, may require content conversion.

A standard SMTP message (also known as a plain-text message) consists of a message envelope and the message contents—the message header and message body. These elements are based on plain 7-bit US-ASCII text. When a message contains elements that are not plain US-ASCII text, the elements must be encoded. When dealing with such non-text content, including attachments, MIME is used for encoding. Both Exchange 2007 and earlier versions of Exchange Server use the Base64 algorithm to encode attachments. And the disadvantage of Base64 is that it bloats attachments by 33%.

In Exchange 2007, except when Outlook Web Access is used, most transport-related content conversion is performed on the Hub Transport server. For a detailed explanation, see the topic "Understanding Content Conversion" at technet.microsoft.com/library/bb232174.


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more info:

http://office.microsoft.com/en-us/outlook/HA102365941033.aspx