Email 2x size of attachment

Question

When I check the size of a .mov file in the finder it is 10.7 MB. When I add the file to an email Mail says it is 20 MB and the smtp server rejects the file because it is over the limit. Why does the email double in size?

Answer 1

When an attachment to an e-mail is sent, is converted using a method called "base-64 encoding" into plain text (electronic mail doesn't handle binary, just text). Using that method, every 6 bytes of data in your file is represented as as string of 8 printable (human readable) characters. Additionally, and end-of-line character is added after the 80th character. There's also a little bit that goes before the attachment and after it to indicate that there's an attachment and where the attachment starts and ends.
The practical upshot is that an attachment is ALWAYS just a little more than 33% bigger than the file it was created from.
Given that, your 10.7M attachment would probably balloon to 15,400,000,000 bytes or so. If the attachment was included in-line (you drag-and-drop). I don't know where the rest of it comes from. The headers for the e-mail message aren't likely to exceed 2K, and the HTML in the message (if sending as rich text) is only likely to increase the size of the text part maybe 1K + 2x the amount of text you put in there (and, I take it you didn't include a megabyte of text).