You are purchasing a Space Email account. This grants you access to some more features which are outlined on the purchase page.
Right now I'm working on gaining some funding to create a mobile version of Space Email for iOS and Android. I don't have any of the development tools. Plus, I need to make a living.
That's fine, it's completely free to use. Check back often, though, there's constant work on adding new things. There's no guarantee the price won't go up, so purchase sooner rather than later, if you can!
Paid users can select the color their messages show up as on the inbox.
Go to "Options" on the compose panel.
No. That's not really the spirit of Space Email.
It only gives you the next one once the first click has processed. So if the loading icon is still there, wait until it's finished before pressing again. This decreases the load on the server. If thing's never come through, and the loading animation stays, then something's up - drop a line from the Contact page or Tumblr.
There is either no subject, no sender, or no body. Otherwise, something is up and you should contact the site.
The current limitation for messages is 1 minute per message. This decreases the load on the server and prevents spam. When things improve, it may go down. If you've waited over a minute and it still says this, try re-loading the page.
It opens in a pop-up window and may be blocked by your pop-up blocker.