REQUIRED SOFTWARE ADD-ON FOR APPLE MAIL
To properly support an HTML email signature, Apple Mail requires the installation of a third-party software package - "Signature Profiler" - it can be purchased for $12 here (or you can install a free 30 day trial version): http://littleknownsoftware.com/sigpro

If you have over 12 users of Apple Mail, we recommend the purchase of a Signature Profiler "Site License" for a one-time fee of $150 - this is for an unlimited number of users and can be purchased from this link: http://sites.fastspring.com/littleknownsoftware/product/sigpro

Be sure to INSTALL the Signature Profiler software before continuing.

Notes about using Signature Profiler with Apple Mail signatures:

HTML signatures have never been easy to install in Apple Mail. What's more, Apple Mail rarely interprets and displays HTML layouts in any reliable way. After lengthy research, we have found the third-party software, Signature Profiler, to be the only reliable and fool-proof solution to the Apple problem. With Signature Profiler, your signature can be installed in a few easy steps and will function with excellent results.

If you have an Apple one-button mouse, click while holding the option or alt key and you get the same result as a right-click.

The terms "directory" and "folder" both mean the same thing.

Mac Mail (OSX) - Email Signature Installation Instructions
Attention: Begin by typing your email address in the field below and click the "Click to Display" button to display your signature in Safari now.

  
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1. With your signature displayed in Safari, highlight the signature's web address in the top address bar. Copy (CMD-C) this address to your clipboard.

Minimize the Safari window and open Apple Mail.
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2. In Apple Mail, open Mail preferences under the Mail drop-down menu (or hit Command-,) - that's Command-"comma".
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3. Go to the Signatures tab. Highlight your email account and click [+] to create a new signature. You can rename the new signature if needed.
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4. Right-click anywhere in the signature box to find the Signature Profiler options (SigPro Replacements). Click Replace with Web Page....
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5. A dialog will appear with "Enter the URL for Your Signature Page". Paste (CMD-V) the signature URL (from STEP 1) into the field. Click Insert.
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6. Your signature will now appear in the signature box although the images will not yet be visible - this is normal and the complete signature will appear when you open a new email.
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ADDITIONAL NOTES:

If the signature appears with some misalignments or gaps, there is nothing wrong. This sometimes happens when composing a new email. Your signature will appear perfectly when delivered to your recipient.

Hyperlinks in email will not work while you are composing or responding to a message.  To test your links, click while holding down the Command key.