I highly recommend Mixmax (admittedly biased; co-founder here :-P). It offers a ton of free features beyond just scheduling emails. You also get unlimited email

I highly recommend Mixmax (admittedly biased; co-founder here :-P). It offers a ton of free features beyond just scheduling emails. You also get unlimited email tracking for free and can put message enhancements in your email (things like link previews, polls, surveys, meeting invites).