Can no longer save email messages to my files

Question

Before upgrading to Mavericks, I could open an email message from a client, click Save As, select a folder, then save the message as a text file to the appropriate folder on my hard drive. It seems I can no longer do that. I always get a Mail error message saying "Names longer than 31 characters are not supported on the destination volume."(Even when the name is already shorter than that, I go through the exercise of shortening it.) After doing that, I click Save. Then I get an error saying "Error: Could not save to path...." which, incidentally, goes haywire part of the way through and takes it down a path that is a completely wrong folder for another client. Regardless, it doesn't save.
I've tried completely renaming the file. I've tried everything I can think of.
This used to be SO SIMPLE:  Just click Save As, select the appropriate folder, then click Save.
In Mavericks, do I need to convert this email message into a Word document in order to save it to a folder?

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.