Email attachment

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.

Current usage

Size limits

Email standards such as MIME 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 agents 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.

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), 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.

History, and technical detail

Originally Internet SMTP email was 7-bit ASCII 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.

The first email attachment was sent by Nathaniel Borenstein on March 11, 1992.