Qualcomm 4.3 user manual Mime and Mapping, What Is MIME?, Mime Encoding

MIME and Mapping

What Is MIME?

MIME stands for Multipurpose Internet Mail Extensions. MIME serves two major purposes

—it allows mail applications to tell one another what sort of data is in mail, and it also provides standard ways for mail applications to encode data so that it can be sent through the Internet mail system.

MIME Encoding

The Internet uses the SMTP protocol to move mail around. SMTP is limited to the US-ASCII character set (see the “Mail Transport” section of this manual). This is a problem for people who speak languages other than American English and so need accented characters or non-American English letters, or for people who want to use special symbols like the bullet.

MIME provides a way around this restriction. It offers two encodings, “quoted-printable” and “base64.” These encodings use US-ASCII character codes to represent any sort of data you like, including special characters or even non-text data.

Quoted-printable is used for data that is mostly text, but has special characters or very long lines. Quoted-printable looks just like regular text, except when a special character is used. The special character is replaced with an “=” and two more characters that repre- sent the character code of the special character. So, a bullet in quoted-printable looks like “=95.”

However, there are some other things that quoted-printable does. For one, since it uses an “=” to mean something special, equals signs must themselves be encoded (as “=3D”). Second, no line in quoted-printable is allowed to be more than 76 characters long. If your mail has a line longer than 76 characters, the quoted-printable encoding will break your line in two, and put an “=” at the end of the first line, to signal to the mail reader at the other end that the two lines are really supposed to be one. Finally, a few mail systems either add or remove spaces from the ends of lines. So, in quoted-printable, any space at the end of a line gets encoded (as “=20”) to protect it from such mail systems.

Let’s try an example. Here’s a passage of text that you might type on your computer:

«Il est démontré, disait-il, que les choses ne peuvent être autrement; car tout étant fait pour une fin, tout est nécessairement pour la meilleure fin.»

Without any encoding, this might show up on your recipient’s screen as:

+Il est dimontri, disait-il, que les choses ne peuvent btre autrement; car tout itant fait pour une fin, tout est nicessairement pour la meilleure fin.;

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