Understanding e-mail attachments:

When Exchange sends an e-mail to another host via SMTP, the message size may change due to the encoding used to package it. This is doubly true for messages with attachments, since the only way to send e-mail attachments is to convert them from plain ASCII to MIME or UU-encoding the message.

Even if an attachment is less than the size limits proscribed by Exchange, it may not be accepted because its MIME-encoded or UU-encoded size is too big.

This happens most often when administrators set limits for inbound SMTP mail. An incoming MIME-encoded e-mail with attachments can increase in size anywhere from 30% to 40%, depending on how many separate attachments, line breaks, MIME headers or other non-data elements are in the message.

The exact size can vary, especially since every e-mail system behave a little differently when converting e-mail attachment(s) to MIME.

The same scenario applies when sending an e-mail with attachment(s), where message sent from your domain would be constrained by message size limit on recipient host(s). Likewise, an e-mail with attachment(s) sent from your domain is going to expand anywhere from 30% to 40% in size when converted.



Useful Links:

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

Base64 Encoding:        http://en.wikipedia.org/wiki/Base64

Plain text base64 calculator: http://www.p23.nl/projects/base64/