I've tested this on 2.8.4, works fine, but would like an enhancement.
The encoder also encodes the & character. Thus when using multiple parameters in a mail to (e.g., subject and body), the encoded & character seems to kill off the remaining parameters.
Here's the mailto I'm trying to use:
mailto:?subject=here is a subject&body=here is a body
The mailto above works fine with dbd-mailto-encoder disabled, but encoding the & seems to make it not work.
I've only tested with Firefox and Thunderbird.