Home : Internet Terms : Attachment Definition


An attachment, or email attachment, is a file sent with an email message. It may be an image, video, text document, or any other type of file.

Most email clients and webmail systems allow you to send and receive attachments. To send an attachment along with your email, you can use the "Attach" command, then browse to the file you want to attach. In some email interfaces, you can simply drag a file into the message window to attach it. When you receive an attachment, most email programs allow you to view the attachment in place or save it to your local storage device.

While modern email programs make it easy to send and receive attachments, the original email system (SMTP) was actually not designed to handle binary files. Therefore, attachments must be encoded as text in order to be transferred with an email message. The most common encoding type is MIME (Multi-Purpose Internet Mail Extensions). While MIME encoding makes it possible to send messages with emails, it typically increases the file size of the attachment about 30%. That's why when you attach a file to an email message, the file size of the attachment appears larger than the original file.

You can attach multiple files to a single email message. However, the maximum size of the combined attachments is limited by the sending and receiving mail servers. In other words, the size of the attachment(s) after being encoded cannot be larger than the limit of either the outgoing or incoming mail server. In the early days of email, attachments were limited to one megabyte (1 MB). Today, many mail servers allow attachments larger than 20 MB. However, to protect against viruses and malware, many mail servers will not accept executable file types, such as .EXE or .PIF files. If you need to send an executable file to someone, you can compress the file as a .ZIP archive before attaching it to the email message.

NOTE: Even a large amount of text takes up a small amount of space compared to most binary files. Therefore, attaching a document to an email may increase the size substantially. For example, a typical email may only require one kilobyte (1 KB) of disk space. Attaching a single 1 MB file will make the message 1,000 times larger. Therefore, it is best to share large files using another method like FTP or DropBox. Additionally, if you have almost reached your email quota on your mail server, you can free up a lot of space by deleting old attachments.

Updated: April 24, 2015

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