What is the encoding that uses an = then 2 hex digits for some non-alphanumeric characters?

I get e-mails that use =3D for =, =92 for ', =20 for a space... The headers say they are ISO-8859-1 but many of the characters they translate are ASCII. I know that 3D is hex for =, 20 hex for space, but they don't translate all spaces.

What is the name for this practice? Why use it? Is there already a program that translates them to ISO-8859 or ASCII? I've written a script so can live without it.