@Vagabond the problem is that [some DKIM tags](http://tools.ietf.org/html/rfc6376#section-3.5) (namely: `i` and `z`) require [dkim-quoted-printable](http://tools.ietf.org/html/rfc6376#section-2.11) which have some minor differences from `encode_quoted_printable`, plus encoding `;` as `=3B`.
`i` tag has e-mail address syntax (are `;` allowed in e-mail address?), `z` is very rare used (see below).

There are 3 ways how this may be solved: 1'st is to add some options to `encode_quoted_printable`, 2'nd is to do some post-processing of `encode_quoted_printable` output and 3-rd is to implement `encode_dkim_quoted_printable` from scratch).
I just choose 4-th variant: ignore rare cases, but make compact implementation, that fits 99% cases.