Please note: this is a fairly rare occurrence and you should examine the methods in this article first, paying attention to the part about “Detect and maintain account settings”;
If you host with me, it is worth running this past me before taking this step if you are not totally confident.
If Apple Mail is not sending mail, it may be an issue todo with the Sandbox folders (don’t ask ;-). It is worth trying this procedure to clear the problem;
Thanks to Linc Davis from this article.
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 with Time Machine.
Triple-click anywhere in the line below on this page to select it:
Right-click or control-click the highlighted line and select Services > Reveal from the 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:
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 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). Press return.