As someone who uses the mac mail client & has been jealous of boomerang for gmail for a while I'm wondering if the computer needs to be on (and if mail needs to be open) for the messages to send. Their site didn't really clarify that point
@paulprins SendLater requires the computer to be on to send scheduled emails. But modern Macs support a feature called 'Power Nap', which shortly wakes your computer to perform tasks such as sending your emails. Thus, SendLater can also send scheduled emails while your computer is asleep.
Mailbutler