Why is a message with an attachment so much larger than the attachment?
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Updated:August 12, 2014
Environment:Cisco Email Security Appliance (ESA), all versions of AsyncOS
You may observe that an email message with a binary attachment appears to be much larger in size when viewed via the mail_logs compared to the size of the binary attachment. This is because binary files in attachments are encoded before the email message is delivered to an MTA. The most popular are base64 and uuencode. The goal of encoding is to accurately represent 8-bit characters within the 7-bit limitation of SMTP.
Take base64 for example: Base64 encoding takes three bytes, each consisting of eight bits, and represents them as four printable characters in the ASCII standard. Therefore a message that appears on an MTA as 100 bytes, is actually 75 bytes in binary format.