An email attachment is a
computer file
A computer file is a computer resource for recording data in a computer storage device, primarily identified by its file name. Just as words can be written to paper, so can data be written to a computer file. Files can be shared with and trans ...
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.
History, and technical detail
Originally, ARPANET, UUCP, and Internet
SMTP
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients ty ...
email allowed
7-bit ASCII text only. Text files were emailed by including them in the message body. In the mid 1980s text files could be grouped with
UNIX
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...
tools such as bundle and
shar
In the Unix operating system, shar (an abbreviation of ''shell archive'') is an archive format created with the Unix shar utility. A shar file is a type of self-extracting archive, because it is a valid shell script, and executing it will re ...
(shell archive) and included in email message bodies, allowing them to be unpacked on remote UNIX systems with a single shell command.
The COMSYS/MSGDMS system at MIT offered "Enclosures" beginning by 1976. Users inside COMSYS could receive the enclosure file directly. Messages sent to users out of the COMSYS world sent the enclosure as part of the message body, which was useful only for text files.
Attaching non-text files was first accomplished in 1980 by manually encoding 8-bit files using
Mary Ann Horton
Mary Ann Horton (born Mark R. Horton, on November 21, 1955), is a Usenet and Internet pioneer. Horton contributed to Berkeley UNIX (BSD), including the vi editor and terminfo database, (see Acknowlegments section at end of file) created the firs ...
's
uuencode, and later using
BinHex or
xxencode and pasting the resulting text into the body of the message. When the "Attachment" user interface first appeared on PCs in
cc:Mail around 1985, it used the
uuencode format for SMTP transmission, as did
Microsoft Mail later.
Modern email systems use the
MIME
Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...
standard, making email attachments more utilitarian and seamless. This was developed by
Nathaniel Borenstein
Nathaniel S. Borenstein (born September 23, 1957) is an American computer scientist.
He is one of the original designers of the MIME protocol for formatting multimedia Internet electronic mail and sent the first e-mail attachment.
Biography
Bore ...
and collaborator
Ned Freed[Father of the email attachment]
Patrick Kingsley, ''The Guardian'', 26 March 2012 - with the standard being officially released a
RFC2045in 1996.
With MIME, a message and all its attachments are encapsulated in a single
multipart message, with
base64 encoding used to convert binary into 7-bit ASCII text - or on some modern mail servers, optionally
full 8-bit support via the
8BITMIME
The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typi ...
extension.
Size limits
Email standards such as
MIME
Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...
do not specify any file size limits, but in practice email users will find that they cannot successfully send very large files across the Internet.
This is because of a number of potential limits:
* Mail systems often arbitrarily limit the size their users are allowed to submit.
* A message will often pass through several
mail transfer agent
The mail or post is a system for physically transporting postcards, letters, and parcels. A postal service can be private or public, though many governments place restrictions on private systems. Since the mid-19th century, national postal syst ...
s to reach the recipient. Each of these has to store the message before forwarding it on, and may therefore also impose size limits.
* The recipient mail system may reject incoming emails with attachments over a certain size.
The result is that while large attachments may succeed internally within a company or organization, they may not when sending across the Internet.
As an example, when
Google
Google LLC () is an American Multinational corporation, multinational technology company focusing on Search Engine, search engine technology, online advertising, cloud computing, software, computer software, quantum computing, e-commerce, ar ...
'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''".
Also note that all these size limits are based, not on the original file size, but the
MIME
Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Message ...
-encoded copy. The common
Base64 encoding adds about 37% to the original file size, meaning that an original 20MB file could exceed a 25MB file attachment limit.
A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.
Malware
A lot of
malware
Malware (a portmanteau for ''malicious software'') is any software intentionally designed to cause disruption to a computer, server, client, or computer network, leak private information, gain unauthorized access to information or systems, depr ...
is distributed via email attachments with some even considering such to be the main
vector
Vector most often refers to:
*Euclidean vector, a quantity with a magnitude and a direction
*Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism
Vector may also refer to:
Mathematic ...
for cyberattacks on businesses.
Users are advised to be extremely cautious with attachments and to not open any attachments that are not from a trusted source and expected − even if the sender is in their address book as their account might have been taken over or misused.
[ While many email servers scan attachments for malware and block dangerous filetypes, this should not be relied upon − especially as such cannot detect zero-day exploits.
]
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 creating mischief and mayhem as early as 1987 with the mainframe-based Christmas Tree EXEC
Christmas Tree EXEC was the first widely disruptive computer worm, which paralyzed several international computer networks in December 1987. The virus ran on the IBM VM/CMS operating system.
Written by a student at the Clausthal University of ...
.
Since the ILOVEYOU and Anna Kournikova
Anna Sergeyevna Kournikova ( rus, Анна Сергеевна Курникова, p=ˈanːə sʲɪrˈɡʲejɪvnə ˈkurnʲɪkəvə, a=Anna_kournikova.ogg; born 7 June 1981) is a Russian former professional tennis player and American televisi ...
worms Worms may refer to:
*Worm, an invertebrate animal with a tube-like body and no limbs
Places
*Worms, Germany
Worms () is a city in Rhineland-Palatinate, Germany, situated on the Upper Rhine about south-southwest of Frankfurt am Main. It had ...
of 2000 and 2001, email systems have increasingly added layers of protection to prevent potential malware
Malware (a portmanteau for ''malicious software'') is any software intentionally designed to cause disruption to a computer, server, client, or computer network, leak private information, gain unauthorized access to information or systems, depr ...
. Now, many block certain types of attachments.''"You may receive an "Outlook blocked access to the following potentially unsafe attachments" message in Outlook"'', microsoft.com
References
{{reflist
Email