I know that it's difficult to implement true send later functionality without dedicated servers, but I would love it if the next time I am online and during work hours, thunderbird automatically sends my outbox emails. I think this should be easier to implement since thunderbird has similar functionality already for going online.
Another alternative would be to allow users to send all emails in outbox using cli commands. This would enable users to create a simple cron job to automate sending emails during specific hours.