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 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.