Under the L-ASA (CRJ) contract, reserves can't pick up flying on a day off because you're already down to minimum days off. You can however have a soft-day (non-golden day) taken away and replaced later in the month. I didn't receive the email, but maybe they meant reserves could volunteer to have their day off taken away and replaced? Not sure what kind of incentive they could offer though, other than normal credit. IMO, the only time it's poor form to volunteer for flying is when people are furloughed or threatened with furlough.