Below every email you write in ZyfrHub (and every reply) there are two options.
Send later
Change Send now to tomorrow morning, tomorrow afternoon, Monday morning, or pick a date and time. The button changes to Schedule. The email goes out from your mailbox at that time, even if you're offline.
See and change scheduled emails in Mail › Scheduled: send one straight away or cancel it.
Remind me if no reply
Pick 1 to 14 days. If the person hasn't replied by then, a Follow up task appears on your activity list and you get a notification. If they reply first, the reminder quietly cancels itself.
Reminders you're waiting on are listed in Mail › Scheduled too.
For a whole series of follow-ups, use a sequence instead.