Need Help with Apple Mail

You would want somebody that's familiar with Kerio Communicator product, and probably less so with the OS X Mail.app client. As a start for troubleshooting this, check the Kerio log files. Also confirm that the server DNS is correct, as that can cause all sorts of weird problems. (Launch Terminal.app on the server and issue the /sudo changeip -checkhostname/ diagnostic command, and see if that reports DNS is correct or if there are DNS errors.) As an alternative, the Kerio folks also offer support for their products.

I've been at this troubleshooting stuff too long to presume a specific culprit, particularly when mail servers are involved. Having worked with both Kerio Communicator a while back and with Apple Mail.app, anyone troubleshooting this is inherently going to be looking at both ends of the mail connection here. If there is another client that works with Kerio, then it may be something weird or corrupted with the specific account, or possibly something weird or corrupted with Apple Mail.app. If there are other clients on other systems that don't work or if the errant credentials from one OS X system don't work when used from another client on another system, then it's probably Kerio. But any testing of Kerio is inherently going to be using both the Kerio server and the Mail.app client, reading the Kerio log files, checking DNS and some related investigation.

Please follow these directions to delete the Mail "sandbox" folder.

Back up all data.

Triple-click the line below 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.

Quit and relaunch 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 (command-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.