An e-mail attachment is a computer file that is sent along with an e-mail message. You can attach different types of files, photographs, zipped folders, etc. An attachment can be an appointment, contact, task, journal entry, note, posted item, and document. Messages that have attachments are identified by a paper clip icon.

Worms and viruses are often distributed as attachments to e-mail messages. With vulnerable e-mail programs the virus may be activated by viewing or previewing the message; more robust programs only allow infection if the user opens the attachment for execution. Unexpected e-mail with attachments should always be considered suspicious and dangerous, particularly if not known to be sent by a trusted source.