Scheduled Reminders do not go to contacts coded as opt out or with bad email messages
The docs on scheduled reminders currently say:
Scheduled reminders are still sent to contacts who have opted out of bulk emails and those who have selected the privacy option Do not email.
However, that functionality was changed and now it seems we don't send scheduled reminders to people who have selected the privacy option "Do not email" (or whose email is on hold but that is not mentioned in the docs). However, we will continue sending to people that have opted out of bulk email messages.