In Mountain Lion, Apple replaced the webarchive format for signatures with a new format .mailsignature.
STEP 1: In Mail, go to Preferences > Signatures and create a signature with any random content. Name it something meaningful. You will be swapping this out later.
Most operating systems and DNS clients will automatically cache IP addresses and other DNS results, this is done in order to speed up subsequent requests to the same hostname. Sometimes bad results will be cached and therefore need to be cleared from the cache in order for you to communicate with the host correctly. All major operating systems allow you to force this process. Outlined below are the common steps you will need to follow in order to flush your DNS cache.
