The MIME specification defines a message format rather than a transport protocol. The format is designed to allow a message in the MIME format to be transported over existing Internet protocols, in particular SMTP.

However, simply following the MIME specification does not guarantee compatibility with older applications. For example, older SMTP servers may not correctly handle a message with data lines longer than 78 characters, even though the MIME specification itself allows data lines of up to 998 characters.

When constructing a MIME part, keep in mind the limitations of the network protocol over which the part will be transmitted. This guide provides advice for creating parts that work within the limitations of SMTP, and points out places where an application can take advantage of less restrictive protocols.