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Introduction to Email 
Multimedia Messages
One of the past limitations
of email is that it was restricted to text. For some time, there has been
a recognised need to be able to send binary data (ie. graphics, word processor
and spreadsheet files). Email containing types of data other than text
is known as multimedia email.
Several solutions have been devised to allow transfer of non-textual information
using email but some of these depend on the same software being employed
at both the sending and receiving system, or conversion software available
on the receiving system. Most of these ad hoc solutions were based on
uuencoding.
The standard way of
dealing with multimedia email is called MIME - Multimedia Internet Mail
Extensions. The non-textual information is supplied in a file and is known
as an attachment..
MIME
MIME is a standard developed as a set of extensions to SMTP - the standard
email protocol in use on Internet. Basically, a set of extensions has
been defined which state how different types of data are to be encoded
as text and included in the message. A separator is defined in the header
and this is used to separate different types of data. A header line is
then included at the start of each section which describes the type of
data in the following section. MIME can use various types of encoding,
but the one normally used for binary files is base64. This encodes the
file into printable text and offers some advantages over uuencode.
This approach has the advantage of allowing multimedia mail to be carried
using existing email services without modification. MIME multimedia mail
is constructed and displayed by suitable user interfaces. If such mail
is received and read using a non-MIME user interface, any encoded sections
of the message are displayed as text and can be extracted and converted
using suitable programs. MIME is supported by many packages, for example
recent versions of Pegasus, Eudora, the Microsoft exchange client on Windows
95 and some versions of elm and pine on Unix.
Problems with MIME
Unfortunately, not all of the problems of multimedia mail are solved by
MIME and some remain unresolved. Not everybody can read MIME messages,
and they tend to be much larger than ordinary text, so are unpopular with
some dial up users because of increased transmission times. Computer viruses
can also be spread using MIME attachments.
Difficulties can also be encountered if you send a word processed document
and the recipient uses different word processing software from you . They
may be unable to make sense of the file even if it is transferred successfully.
If you are using a
MIME mail interface, and include plain text files, it is important that
you do not encode these, as it can create unnecessary difficulties if
the recipient does not use a MIME interface.
Finally, because of
these as yet unresolved problems, encoded messages are unpopular on mailing
lists in general as they tend to be large messages and mailing lists often
contain members with vastly different computing environments. Unless posting
such messages is generally accepted on the list to which you are posting
it is advisable to avoid sending any encoded messages. One popular alternative
is to put the formatted document or graphical image onto a WWW page and
send the address and a brief description of the information to the mailing
list.
Last Updated 18 Aug 2000. Please mail any
comments to C.B.Bayliss@bham.ac.uk
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