Gmail provides a “Schedule send” in order to send an email at a chosen later time. I could not find that feature on Runbox.
There is a workaround using Thunderbird and the Send Later add-on. But that requires Thunderbird to be running at the scheduled time. Apart from that, it seems to be brittle when the local internet connection (and/or power) is failing during/until the scheduled time. I would very much prefer to use a server-side feature for that. Especially, when much depends on it to actually work.
Do you have that feature in some paid plan? If not, do you plan to implement that in a more or less near future?