We understand that our community of email communicators wants more control over determining when emails are sent automatically. At this time, we unfortunately do not have this feature within our service; however, this is a top-level feature request, and we are looking into a variety of options to deliver this feature to Soapbox Mailer in the future. There is not a release date scheduled for this as we're still investigating options, but when there is, we'll be sure to let you know.
Happy emailing!