Email attachment

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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.

Contents

History, and technical detail

Originally, ARPANET, UUCP, and Internet SMTP 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 tools such as bundle [1] [2] and shar (shell archive) [3] 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. [4] [5] 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's uuencode, and later using BinHex or xxencode [6] 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, [7] it used the uuencode format for SMTP transmission, as did Microsoft Mail later.

Modern email systems use the MIME standard, making email attachments more utilitarian and seamless. This was developed by Nathaniel Borenstein and collaborator Ned Freed [8] [9] - with the standard being officially released as RFC2045 in 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 extension.

Size limits

Email standards such as MIME 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:

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'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". [11] [12]

Also note that all these size limits are based, not on the original file size, but the MIME-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. [13] A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.

You should also avoid certain different types of file formats as email attachments for example, .zip or .tgz files because it can contain harmful viruses and potential software, .iso file because they can also be used to spread malware and .exe is an executable file that can become active on your computer as soon as you open it. [14]

Malware

A lot of malware is distributed via email attachments with some even considering such to be the main vector for cyberattacks on businesses. [15] [16] [17] 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. [15] [18] [19] 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. [20]

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.

Since the ILOVEYOU and Anna Kournikova worms of 2000 and 2001, email systems have increasingly added layers of protection to prevent potential malware. Now, many block certain types of attachments. [21] [22]

Related Research Articles

<span class="mw-page-title-main">Email</span> Mail sent using electronic means

Electronic mail is a method of transmitting and receiving messages using electronic devices. It was conceived in the late–20th century as the digital version of, or counterpart to, mail. Email is a ubiquitous and very widely used communication medium; in current use, an email address is often treated as a basic and necessary part of many processes in business, commerce, government, education, entertainment, and other spheres of daily life in most countries.

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 bodies may consist of multiple parts, and header information may be specified in non-ASCII character sets. Email messages with MIME formatting are typically transmitted with standard protocols, such as the Simple Mail Transfer Protocol (SMTP), the Post Office Protocol (POP), and the Internet Message Access Protocol (IMAP).

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 typically use SMTP only for sending messages to a mail server for relaying, and typically submit outgoing email to the mail server on port 587 or 465 per RFC 8314. For retrieving messages, IMAP is standard, but proprietary servers also often implement proprietary protocols, e.g., Exchange ActiveSync.

<span class="mw-page-title-main">Email client</span> Computer program used to access and manage a users email

An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email.

8-bit clean is an attribute of computer systems, communication channels, and other devices and software, that process 8-bit character encodings without treating any byte as an in-band control code.

Mbox is a generic term for a family of related file formats used for holding collections of email messages. It was first implemented in Fifth Edition Unix.

UUCP is a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers.

<span class="mw-page-title-main">Outlook Express</span> Microsoft e-mail client software

Outlook Express, formerly known as Microsoft Internet Mail and News, is a discontinued email and news client included with Internet Explorer versions 3.0 through 6.0. As such, it was bundled with several versions of Microsoft Windows, from Windows 98 to Windows Server 2003, and was available for Windows 3.x, Windows NT 3.51, Windows 95, Mac System 7, Mac OS 8, and Mac OS 9. In Windows Vista, Outlook Express was superseded by Windows Mail.

In computer programming, Base64 is a group of binary-to-text encoding schemes that transforms binary data into a sequence of printable characters, limited to a set of 64 unique characters. More specifically, the source binary data is taken 6 bits at a time, then this group of 6 bits is mapped to one of 64 unique characters.

Bogofilter is a mail filter that classifies e-mail as spam or ham (non-spam) by a statistical analysis of the message's header and content (body). The program is able to learn from the user's classifications and corrections. It was originally written by Eric S. Raymond after he read Paul Graham's article "A Plan for Spam" and is now maintained together with a group of contributors by David Relson, Matthias Andree and Greg Louis.

<span class="mw-page-title-main">Apple Mail</span> Email client by Apple Inc.

Mail is an email client included by Apple Inc. with its operating systems macOS, iOS, iPadOS, watchOS, and visionOS. Mail grew out of NeXTMail, which was originally developed by NeXT as part of its NeXTSTEP operating system, after Apple's acquisition of NeXT in 1997.

uuencoding is a form of binary-to-text encoding that originated in the Unix programs uuencode and uudecode written by Mary Ann Horton at the University of California, Berkeley in 1980, for encoding binary data for transmission in email systems.

Quoted-Printable, or QP encoding, is a binary-to-text encoding system using printable ASCII characters to transmit 8-bit data over a 7-bit data path or, generally, over a medium which is not 8-bit clean. Historically, because of the wide range of systems and protocols that could be used to transfer messages, e-mail was often assumed to be non-8-bit-clean – however, modern SMTP servers are in most cases 8-bit clean and support 8BITMIME extension. It can also be used with data that contains non-permitted octets or line lengths exceeding SMTP limits. It is defined as a MIME content transfer encoding for use in e-mail.

yEnc is a binary-to-text encoding scheme for transferring binary files in messages on Usenet or via e-mail. It reduces the overhead over previous US-ASCII-based encoding methods by using an 8-bit encoding method. yEnc's overhead is often as little as 1–2%, compared to 33–40% overhead for 6-bit encoding methods like uuencode and Base64. yEnc was initially developed by Jürgen Helbing, and its first release was early 2001. By 2003 yEnc became the de facto standard encoding system for binary files on Usenet. The name yEncode is a wordplay on "Why encode?", since the idea is to only encode characters if it is absolutely required to adhere to the message format standard.

Transport Neutral Encapsulation Format or TNEF is a proprietary email attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most often named winmail.dat or win.dat, and has a MIME type of Application/MS-TNEF. The official (IANA) media type, however, is application/vnd.ms-tnef.

Many email clients now offer some support for Unicode. Some clients will automatically choose between a legacy encoding and Unicode depending on the mail's content, either automatically or when the user requests it.

The Simple Mail Access Protocol (SMAP) is an application layer Internet protocol for accessing email stored on a server. It was introduced as part of the Courier suite, with the goal of creating a simpler and more capable alternative to IMAP.

Email encryption is encryption of email messages to protect the content from being read by entities other than the intended recipients. Email encryption may also include authentication.

<span class="mw-page-title-main">Gmail interface</span> Overview of the interface of Googles email service Gmail

The Gmail interface makes Gmail unique amongst webmail systems for several reasons. Most evident to users are its search-oriented features and means of managing e-mail in a "conversation view" that is similar to an Internet forum.

SMTP Authentication, often abbreviated SMTP AUTH, is an extension of the Simple Mail Transfer Protocol (SMTP) whereby a client may log in using any authentication mechanism supported by the server. It is mainly used by submission servers, where authentication is mandatory.

References

  1. ↑ The UNIX Programming Environment, Kernighan and Pike, 1984, p.97
  2. ↑ "Unix tricks and traps". AUUGN. 15 (4): 87. August 1994.
  3. ↑ Modern versions of shar can deal with binaries, via uuencoding them, but this was not initially the case.
  4. ↑ "Jack Haverty, email to Header-People, 8 November 1976"
  5. ↑ "Feinler, Vittal: Email Innovation Timeline, 1 July 2022"
  6. ↑ "How do I use UUencode/BinHex/MIME support?", winzip.com.
  7. ↑ InfoWorld Media Group, Inc. (June 3, 1985). InfoWorld. InfoWorld Media Group, Inc. p. 41.
  8. ↑ Father of the email attachment, Patrick Kingsley, The Guardian, 26 March 2012
  9. ↑ "The MIME guys: How two Internet gurus changed e-mail forever " Archived 2012-01-25 at the Wayback Machine , February 01, 2011, Jon Brodkin, Network World
  10. ↑ "Setting Message Size Limits in Exchange 2010 and Exchange 2007";
  11. ↑ "Google updates file size limits for Gmail and YouTube", geek.com Archived 2011-12-19 at the Wayback Machine .
  12. ↑ "Maximum attachment size", mail.google,com.
  13. ↑ "Raw vs. Encoded Email Message Size — What's the Difference?".
  14. ↑ "Types Of Email Attachment Format You Should Avoid Sending As An Email Attachment In 2024! | GrowMeOrganic". www.growmeorganic.com. 2021-08-03. Retrieved 2024-03-08.
  15. 1 2 Martin, Jim. "Here's what you need to do to protect your PC from ransomware and NotPetya". Tech Advisor. Retrieved 29 June 2017.
  16. ↑ "Truth on zero-day attacks". PCR. Retrieved 29 June 2017.
  17. ↑ Aycock, John (2006). Computer Viruses and Malware. Springer. ISBN   9780387341880 . Retrieved 29 June 2017.
  18. ↑ Miller, Michael R. (2009). Microsoft Security Essentials User Manual (Digital Short Cut). Pearson Education. ISBN   9780768695298 . Retrieved 29 June 2017.
  19. ↑ Vermaat, Misty E. (2014). Enhanced Discovering Computers, Essentials. Cengage Learning. ISBN   9781285845531 . Retrieved 29 June 2017.
  20. ↑ "How To Spot A Dangerous Email Attachment". MakeUseOf. Retrieved 29 June 2017.
  21. ↑ "Some file types are blocked", mail.google.com.
  22. ↑ "You may receive an "Outlook blocked access to the following potentially unsafe attachments" message in Outlook", microsoft.com.
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