E-mail attachment
Encyclopedia
An email attachment is a computer file
sent along with an email message
. One or more files can be attached to any email message, and be sent along with it to the recipient. This is typically used as a simple method to share documents and images. A paper clip
image is the standard image for an attachment in an email client.
don't specify any file size limits, but in practice email users will find that they can't send very large files.
Over the Internet
a message will often pass through several mail transfer agent
s to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore need to impose size limits. The result is that while large attachments may succeed internally within a company or organization, they are unreliable when sending across the Internet – and for that reason sending systems often arbitrarily limit the size their users are allowed to submit. As an example, when Google
's gmail
service increased its arbitrary limit to 25MB it warned that: "you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits".. In general, 10MB is considered safe for the maximum size of an email.
Email users can be puzzled by these limits because the MIME
encoding adds up to 30% overhead – so that a 20MB document on disk exceeds a 25MB file attachment limit.
However, in practice this advice is not enough – "known trusted sources" were the senders of executable programs
creating mischief and mayhem as early as 1987 (with the mainframe-based Christmas Tree EXEC
), so since the ILOVEYOU
and Anna Kournikova
worms
of 2000 and 2001 email systems have increasingly added layers of protection to prevent potential malware
– and now many block certain types of attachments.
text only, and attaching files was done by manually encoding 8-bit files using uuencode
, BinHex
or xxencode
and pasting the resulting text into the body of the message. The MIME
standard introduced by RFC2045 in 1996 provided a framework to encapsulate a message and all its attachments in a single multipart message, and standardised on base64
encoding to convert binary into 7-bit ASCII. Modern mail servers running Extended SMTP
have the option of advertising full 8-bit support
via the 8BITMIME extension.
Computer file
A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...
sent along with an email message
Email
Electronic mail, commonly known as email or e-mail, is a method of exchanging digital messages from an author to one or more recipients. Modern email operates across the Internet or other computer networks. Some early email systems required that the author and the recipient both be online at the...
. One or more files can be attached to any email message, and be sent along with it to the recipient. This is typically used as a simple method to share documents and images. A paper clip
Paper Clip
"Paper Clip" is a 1995 episode of The X-Files television series. It was the second episode broadcast in the show's third season. Paper Clip concludes the story regarding the agents' possession of a digital tape containing government secrets on extraterrestrials.- Plot :Continuing from the previous...
image is the standard image for an attachment in an email client.
Size limits
Email standards such as MIMEMIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
don't specify any file size limits, but in practice email users will find that they can't send very large files.
Over the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
a message will often pass through several mail transfer agent
Mail transfer agent
Within Internet message handling services , a message transfer agent or mail transfer agent or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture...
s to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore need to impose size limits. The result is that while large attachments may succeed internally within a company or organization, they are unreliable when sending across the Internet – and for that reason sending systems often arbitrarily limit the size their users are allowed to submit. As an example, when Google
Google
Google Inc. is an American multinational public corporation invested in Internet search, cloud computing, and advertising technologies. Google hosts and develops a number of Internet-based services and products, and generates profit primarily from advertising through its AdWords program...
's gmail
Gmail
Gmail is a free, advertising-supported email service provided by Google. Users may access Gmail as secure webmail, as well via POP3 or IMAP protocols. Gmail was launched as an invitation-only beta release on April 1, 2004 and it became available to the general public on February 7, 2007, though...
service increased its arbitrary limit to 25MB it warned that: "you may not be able to send larger attachments to contacts who use other email services with smaller attachment limits".. In general, 10MB is considered safe for the maximum size of an email.
Email users can be puzzled by these limits because the MIME
MIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
encoding adds up to 30% overhead – so that a 20MB document on disk exceeds a 25MB file attachment limit.
Dangerous file types
Email users are typically warned that unexpected email with attachments should always be considered suspicious and dangerous, particularly if not known to be sent by a trusted source.However, in practice this advice is not enough – "known trusted sources" were the senders of executable programs
Computer program
A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...
creating mischief and mayhem as early as 1987 (with the mainframe-based Christmas Tree EXEC
Christmas Tree EXEC
Christmas Tree EXEC was the first widely disruptive computer worm, which paralyzed several international computer networks in December 1987.Written by a student at the Clausthal University of Technology in the REXX scripting language, it drew a crude Christmas tree as text graphics, then sent...
), so since the ILOVEYOU
ILOVEYOU
ILOVEYOU, also known as Love Letter, is a computer worm that successfully attacked tens of millions of computers in 2000 when it was sent as an attachment to a user with the text "ILOVEYOU" in the subject line. The worm arrived e-mail on and after May 4, 2000 with the simple subject of "ILOVEYOU"...
and Anna Kournikova
Anna Kournikova (computer virus)
The Anna Kournikova computer virus was a computer virus authored by Dutch programmer Jan de Wit on February 11, 2001. It was designed to trick email users into opening a mail message purportedly containing a picture of tennis player Anna Kournikova, while actually hiding a malicious program...
worms
Computer worm
A computer worm is a self-replicating malware computer program, which uses a computer network to send copies of itself to other nodes and it may do so without any user intervention. This is due to security shortcomings on the target computer. Unlike a computer virus, it does not need to attach...
of 2000 and 2001 email systems have increasingly added layers of protection to prevent potential malware
Malware
Malware, short for malicious software, consists of programming that is designed to disrupt or deny operation, gather information that leads to loss of privacy or exploitation, or gain unauthorized access to system resources, or that otherwise exhibits abusive behavior...
– and now many block certain types of attachments.
History, and technical detail
Originally Internet SMTP email was 7-bit ASCIIASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
text only, and attaching files was done by manually encoding 8-bit files using uuencode
Uuencode
Uuencoding is a form of binary-to-text encoding that originated in the Unix program uuencode, for encoding binary data for transmission over the uucp mail system.The name "uuencoding" is derived from "Unix-to-Unix encoding"...
, BinHex
BinHex
BinHex, short for "binary-to-hexadecimal", is a binary-to-text encoding system that was used on the Mac OS for sending binary files through e-mail. It is similar to Uuencode, but combined both "forks" of the Mac file system together, along with extended file information...
or xxencode
Xxencode
Xxencode is an binary-to-text encoding similar to Uuencode which uses only the alphanumeric characters, and the plus and minus signs. It was invented as a means to transfer files in a format which would survive character set translation, particularly that between ASCII and the EBCDIC encoding used...
and pasting the resulting text into the body of the message. The MIME
MIME
Multipurpose Internet Mail Extensions is an Internet standard that extends the format of email to support:* Text in character sets other than ASCII* Non-text attachments* Message bodies with multiple parts...
standard introduced by RFC2045 in 1996 provided a framework to encapsulate a message and all its attachments in a single multipart message, and standardised on base64
Base64
Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation...
encoding to convert binary into 7-bit ASCII. Modern mail servers running Extended SMTP
Extended SMTP
Extended SMTP , sometimes referred to as Enhanced SMTP, is a definition of protocol extensions to the Simple Mail Transfer Protocol standard...
have the option of advertising full 8-bit support
8-bit clean
8-bit clean describes a computer system that correctly handles 8-bit character sets, such as the ISO 8859 series and the UTF-8 encoding of Unicode.- History :...
via the 8BITMIME extension.