E-mail Attachment

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.

An obscure cause of corruption of attachments: It is not widely known that e-mail messages may sometimes be corrupted in transmission through the Internet: lines containing a leading dot may have another dot added. This is invariably due to errors in software processing messages somewhere between sender and recipient, but is not uncommon. This is usually harmless, and unlikely even to be noticed in a text message; and it doen not affect messages encoded using Base64 and other binary protocols. However, Microsoft Outlook Express encodes a message in which about 90% or more of the characters are text as Quoted-printable ; this cannot be overriden. A structured Database file may be more than 90% text; if a leading dot is added anywhere it effectively corrupts the whole file.

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 suspect and dangerous, particularly if not known to be sent ny a trusted source.

Some mail services and software filter out potentially dangerous attachments such as Executable s 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.

Attached files which can be executed in some way ( is the most usual. The recipient will receive the attachment, but must extract the file from the archive before it can be excecuted

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E-mail Attachment