(Last updated: October 29, 2013)
Ah, the mailto tag. The easiest way to let people contact you or your company from your website. Easily readable and clickable... and harvestable. Therein lies the problem. Spammers use email harvesting software to scan pages for email addresses to send their spam messages to. And, of course, the venerable mailto tag is the easiest thing for them to pull the address from.
So, some people started just having their email address on the page as the text version of the email address. You have to copy and paste it into your email program, but it still works. Unfortunately, the harvesting programs where updated to catch that, too.
Then people started encoding their address. Instead of email@example.com, it was emai... in the code. Still readable and copyable, but not clickable. And yet again, it appears that the spam filters have caught up and are able to harvest these encoded addresses.
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