Email size different from total attachment size

[Myke 0]

This is more of a curiosity question. I've noticed that when an e-mail with attachments arrives from an external source, the e-mail's total size will be significantly larger than the combined total of the attachments. For instance, an employee received an e-mail with a size of 10 MB. It contained 3 attachments, each being only 1 MB (total of 3 MB). There is little or no text, but the message is coded in HTML format.

 

When the user forwarded the e-mail to me (internally), the e-mail's size was only 4 MB, which makes much more sense. I had originally thought that perhaps this was due to encryption, which doesn't make a whole lot of sense, but was the only thing I could think of. The originally message was also sent to 3 other people, besides our user.

 

Does anyone have any clue to why this is the case and what is causing it?

[peterh 1]

Most attachments are binary files, to send them as an Email attachment they have to be converted to BASE64 (an ASCII based format) so that they can be sent using Email, as none printable ASCII characters, except CR and LF characters, are not allowed. This can substantially increase the size of the message!

 

See http://en.wikipedia.org/wiki/MIME

 

[Myke 0]

Thanks for the input. I ended up finding another reason for this. Apparently, e-mail formats can sometimes skew the file size of an attachment, mainly HTML and MIME formats. They have to potential to bloat the attachment's size from 2/3 to 2 times the original size. This is what was causing this to happen on the network. We've had to advise certain persons to the reasoning why some e-mails are getting bounced.