Can I snooze mail, or send it later?
Both work; one of them depends on an app being open, and it says which.
Snoozing moves a message into a Snoozed folder and brings it back when you asked for it. The wake time is stored on the server, so every device agrees about it — but a CanopyMail app has to be running to bring it back. If everything is closed all weekend, it returns when you next open one.
Sending later works one of two ways depending on your mail server. If the server can hold a message, it will send at the time you chose whether or not your devices are on. If it cannot, the message waits in this app instead. The composer tells you which of the two you are getting, before you commit.
We would rather say this plainly than let you find out by missing a send.