"Sending an email" is something the user initiates. "Syncing to iCloud" is something the OS initiates.
Also, I would hope that "sending an email and closing the browser window", one day, will work like "sending an email and quitting the mail app" works today.
Power Nap is meant to do this - keep stuff syncing, backing up, etc regardless of lid state. Now, whether it's smart enough to flush any synchronizations that are still in progress when the lid is closed or you have to wait a few minutes until it half-wakes-up and does its thing, I'm not sure. Also, I believe by default it's off when a power supply isn't attached.
Also, I would hope that "sending an email and closing the browser window", one day, will work like "sending an email and quitting the mail app" works today.