Losing messages in Mail - Mountain Lion & Lion

Question

I am continuously losing messages in Mail/OS X Lion and Mountain Lion, but only in my Apple ID mail-account.
I will receive a message, see it arrive in the mailbox for the account, and within a couple of seconds it seemingly disappears into the great cyber-void.
The message is nowhere to be found on my machine and also not in iCloud, when logging in there.
I have no rules set, except for the one from Apple, which comes with Mail.
I am, temporarily I hope, able to read the messages, because I in iCloud set Mail to copy all messages to a gmail-account of mine.
But for that, I'd be lost!
I am using Mail extensively for business, so this is a MAJOR problem.
My set-up is:
Running a home-network with a Mac Pro and a Macbook Pro, both running mail.
I have two routers, an Apple Extreme connected to the service-provider router, and an Apple Extreme Dual running in bridge mode.
I also connect via wireless from both an iPhone and an iPad.
Any help would be very, very much appreciated.
Also, how do I get Apple to listen and reply?
Thanks very much in advance!

Answer 1

Please follow these directions to delete the Mail "sandbox" folder.
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. 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. Post your results.
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 combinationcommand-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.