Cisco Cisco Email Security Appliance X1050 Guia De Informação
Why are mail attachments bigger than the original
file?
file?
Document ID: 118499
Contributed by Mark Strasheim and Robert Sherwin, Cisco TAC
Engineers.
Engineers.
Oct 09, 2014
Contents
Introduction
Why are mail attachments bigger than the original file?
Related Information
Why are mail attachments bigger than the original file?
Related Information
Introduction
This document describes email size limit with attachments and why an actual email size may be larger than
expected, when processing through a Cisco Email Security Appliance (ESA).
expected, when processing through a Cisco Email Security Appliance (ESA).
Why are mail attachments bigger than the original file?
MIME (Multipurpose Internet Mail Extensions) specification, defined in RFC 2045, lists "base64" as one of
several binary−to−text encoding schemes. MIME's base64 encoding is based on that of the RFC 1421 version
of privacy enhanced mail (PEM), it uses the same 64−character alphabet and encoding mechanism as PEM,
and uses the "=" symbol for output padding in the same way.
several binary−to−text encoding schemes. MIME's base64 encoding is based on that of the RFC 1421 version
of privacy enhanced mail (PEM), it uses the same 64−character alphabet and encoding mechanism as PEM,
and uses the "=" symbol for output padding in the same way.
MIME does not specify a fixed length for base64−encoded lines, but it does specify a maximum length of 76
characters. Additionally it specifies that any extra−alphabetic characters must be ignored by a compliant
decoder, although most implementations use a CR/LF newline pair to delimit encoded lines.
characters. Additionally it specifies that any extra−alphabetic characters must be ignored by a compliant
decoder, although most implementations use a CR/LF newline pair to delimit encoded lines.
Thus, the actual length of MIME−compliant base64−encoded binary data is usually about 137% of the
original data length, though for very short messages the overhead can be a lot higher because of the overhead
of the headers. Very roughly, the final size of base64−encoded binary data is equal to 1.37 times the original
data size + 814 bytes (for headers).
original data length, though for very short messages the overhead can be a lot higher because of the overhead
of the headers. Very roughly, the final size of base64−encoded binary data is equal to 1.37 times the original
data size + 814 bytes (for headers).
Related Information
Cisco Email Security Appliance − End−User Guides
•
Technical Support & Documentation − Cisco Systems
•
Updated: Oct 09, 2014
Document ID: 118499