Quickmail uses the course context, not the block context, to generate its user list. Because of this, overrides set at the block level do not apply.

The fix for this is very easy; in blocks/quickmail/email.php change line 25 from:

$context = get_context_instance(CONTEXT_COURSE, $course->id);

to:

$context = get_context_instance(CONTEXT_BLOCK, $instanceid);