Enable Power Nap So Scheduled Emails Go Out When Scheduled For Future Date (When Mac is Sleeping)
If you schedule your emails with Send Later (i.e. tomorrow morning at 8 a.m.), the emails aren't sent unless computer is awake and active. If you were able to integrate Power Nap (or sync to online server), emails would go out on schedule, even if computer with Airmail App is sleeping.
John Ruman commented
This is a challenge I've been having and would love it to be corrected somehow.