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
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.
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.
exact size can vary, especially since every e-mail system behave a
little differently when converting e-mail attachment(s) to MIME.
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.
Plain text base64 calculator: http://www.p23.nl/projects/base64/