Mail is getting stuck in my outbox?

Question

Hi
I've got a 13" Macbook Air that's about 12 months old. It's got a 1.7ghz I7 processor and 8GB of RAM.  About 2-3 weeks ago I started having intermittent issues sending email where my mail just sits in the outbox and doesn't send. I'm using the Apple Mail client and have an iCloud email account, although I'm still using the @me.com domain for the email.
I've checked the Connection Doctor. I get green lights all the way through and it says "Connection to server and login succeeded", but my emails still aren't sending. I can't be certain of this but it seems more likely to happen when I send something with an attachment. The emails would be under 10mb still.
If I try quitting mail from Mail>Quit Mail the email application won't close.  if I try to quit by right clicking on it in the menu bar it won't close either. I have to use "Force Quit" off the apple menu to shut the application down.
As an example, looking at the emails stuck in there at the moment there's three. The first one I tried to send at 9:21am. It's now 10:38am and it still hasn't gone. I've checked my internet connection and I'm getting 4mb/sec download and 1.7mb/second upload. So not crazy fast but more than enough to send three emails.
I've tried removing the account and adding it again. This took ages to download all my emails locally again but didn't fix any of the problems.
Any suggestions?

Answer 1

Please follow these directions to delete the Mail "sandbox" folders. In OS X 10.9 there are two sandboxes, while in 10.8 there is only one. If you're running a version older than 10.8, this comment isn't applicable.
Back up all data.
Triple-click anywhere in the line below on this page to select it:
~/Library/Containers/com.apple.mail
Right-click or control-click the highlighted line and select
Services ▹ Reveal
from the contextual menu.* A Finder window should open with a folder named "com.apple.mail" selected. If it does, move the selected folder—not just its contents—to the Desktop. Leave the Finder window open for now.
Log out and log back in. Launch Mail and test. If the problem is resolved, you may have to recreate some of your Mail settings. You can then delete the folder you moved and close the Finder window.
This action will delete any custom Mail stationery that you have created. If you want to preserve it, ask for instructions.
If you still have the problem, quit Mail again and put the folder back where it was, overwriting the one that may have been created in its place. Repeat with this line:
~/Library/Containers/com.apple.MailServiceAgent
Caution: If you change any of the contents of the sandbox, but leave the folder itself in place, Mail may crash or not launch at all. Deleting the whole sandbox will cause it to be rebuilt automatically.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination  command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar and paste into the box that opens by pressing command-V. You won't see what you pasted because a line break is included. Press return.