A Liquid Filter for obfuscating an Email Address (can be used with Jekyll aswell).

example_jekyll.md

In Jekyll set a variable for the mail, e.g. in the _config.yml

email: [email protected]

then use it in your page

Reach me under:	{{ site.email | mailObfuscate }}

which will generate the following HTML

<a href="#" data-contact="bmFtZUBtYWlsLmNvbQ== " target="_blank" onfocus="this.href = 'mailto:' + atob(this.dataset.contact)">    
    <script type="text/javascript">document.write(atob("bmFtZUBtYWlsLmNvbQ== "));</script>
</a>

This uses some simple obfuscation techniques of url encode and base64 encode the mail and use JS to support the link and write it to HTML programmatically. This is certainly not bulletproof, but a good shield and in combination with a good spam filter this will fix your problem with mail crawlers.

These techniques are partly from http://techblog.tilllate.com/2008/07/20/ten-methods-to-obfuscate-e-mail-addresses-compared/

mail_obfuscate.rb

require "base64"
require "uri"

module ObfuscateMailAddress
  def mailObfuscate(input)
    base64Mail = Base64.strict_encode64(Addressable::URI.encode(input))

    # See http://techblog.tilllate.com/2008/07/20/ten-methods-to-obfuscate-e-mail-addresses-compared/
    output = "<a href=\"#\" "
    output += "data-contact=\"#{base64Mail}\" target=\"_blank\" "
    output += "onfocus=\"this.href = 'mailto:' + atob(this.dataset.contact)\">"
    output += "<script type=\"text/javascript\">document.write(atob(\"#{base64Mail}\"));</script></a>"
    return output
  end
end

Liquid::Template.register_filter(ObfuscateMailAddress)

The error mentioned above by @tim-taylor can be resolved by changing line #6 from

base64Mail = Base64.strict_encode64(URI::encode(input))

to

base64Mail = Base64.strict_encode64(Addressable::URI.encode(input))

Thanks very much @digitigradeit! That does indeed solve the problem.