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.
Email standards such as "MIME don't specify any file size limits, but in practice email users will find that they can't 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".
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. A 10MB email size limit would require that the size of the attachment files is actually limited to about 7MB.
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 – and now many block certain types of attachments.
Originally Internet "SMTP email was "7-bit ASCII text only. Text files coulld be emailed by simply including them in the message body, but attaching non-text files was done by first encoding them to 7-bit ASCII text. To do this the "shar utility, "Mary Ann Horton's "uuencode, and later "BinHex or "xxencode could be used; and the resulting text pasted into the body of the message.
Modern email systems use the "MIME standard, making email attachments more utilitarian and seamless. This was developed by "Nathaniel Borenstein and collaborator "Ned Freed; with the first MIME email attachment being sent by "Nathaniel Borenstein on March 11, 1992 and 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 to convert binary into 7-bit ASCII - or on modern mail servers, optionally "full 8-bit support via the "8BITMIME extension.
The "Attachment" user interface first appeared on PCs in "cc:Mail around 1985, using the "uuencode format for SMTP transmission. Later, "Microsoft Mail also used uuencode for SMTP transmission of attachments.