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Why does an email's size increase when sent externally?

Learn what is causing an increase in message size when an email with an attachment is sent by an Outlook MAPI client from an Exchange 2003 server to an external Internet address.

What causes the 30% increase in message size when an email with an attachment is sent by a Microsoft Outlook MAPI client (from an Exchange 2003 server) to an external Internet address? Are there any guidelines to estimate the size increase various document types will experience, (i.e., .pdf, .doc, .xls, .ppt, etc.)?
I'm pretty sure that MIME encoding accounts for the message size difference you are seeing.

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