An e-mail attachment (or email attachment) is a computer file which is sent along with an e-mail message. The file is not a separate message, but now it is almost universally sent as part of the message to which it is attached. Attached messages may be sent in unencoded form, or encoded in a number of ways: base64, binhex, uuencoding, quoted-printable. In MIME, the standard Internet e-mail format, messages and their attachments are sent as a single multipart message, usually using base64 encoding for non-text attachments.

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.

Some mail services and software filter out potentially dangerous attachments such as executables and scripts, although more expert users may find this limitation a nuisance. Viruses in attachments to or the body of e-mail may be scanned for and dealt with by anti-virus software running on the host computer, mail client software, and mail and Internet service providers, although non-detection of a virus does not guarantee a message to be safe.

Mail services have a limit on the size of messages which may be sent and received; this limit may restrict the size of files to be attached. Messages of excessive size will usually be returned to the sender as undeliverable. This usually happens to attachments with a total size of over 30 MB.

As size of exchanged documents increase, the current limitation on email attachments prompted the industry to create solutions to deliver large email attachments. A first alternative was to set dedicated ftp servers so as to overcome this size limitation. The problem was that few corporate firewalls allow the ftp protocol, thus limiting the impact of such solutions. Hence following solutions relied on the HTTP protocol to be more firewall friendly and to provide a more accessible interface from the Internet.

Current email attachment replacements support maximum size of 100MB to 2GB. But the size of exchanged files keeps increasing at a very fast rate. Some companies now support delivery of files of unlimited size, using for instance a Java based file transfer agent.

http://www.learnthenet.com/english/html/94attach.htm (How to use email attachments) http://www.microsoft.com/protect/computer/viruses/email.mspx (Risks of email)