Question posted in 
Lots of people seem to recommend hidden preheader text. For those who don’t know, preheader text is a way to control the e-mail content preview, like this:
An example of hiding it would be:
<div style="display:none;font-size:1px;color:#333333;line-height:1px;max-height:0px;max-width:0px;opacity:0;overflow:hidden;">
Wishing you a safe and merry holiday season!
</div>
I’m wondering if hiding this preheader text from humans reading the e-mail might increase spam score or impact deliverability? In the world of web crawlers, hiding content from users but not machines (so-called cloaking) is a big no-no, and it can really hurt you.
Does anyone know if spam checkers might employ similar logic? I’ve seen some conjecture online, but not much in the way of solid references. Any anecdotes, quotes, or links on this topic would be helpful.
2
Answers
Short answer: yes, it can.
More detailed answer: add hidden text is exactly what spammers do, to bypass spam filters. Every spam filter can detect this hidden-zero-height-zero-width text and – depends on its configuration – will take that into account for the spam score calculation. It’s certainly not a single spam marker, but with all the rest of your email, that might brings you over the threshold.
SpamAssassin can be configured to detect that.
I understand, that this is great for marketing purposes, but to get my stuff delivered, I would rather avoid it.
Carsten is right, hiding text in your email and playing with the email client behaviour to get it displayed will increase the spaminess of your email.
I have been working as a developer on antispam filter for several years, hiding text is a very common spammer technique. It is used to outline several words (or even letters) of a block of random text to display the spammer’s message, making it harder for antispams filters to identify a common pattern.
Same way, hiding text between a subject header and the first mime part (another kind of preheader) is a common spammer technique.
Alone, these criterias may not be enough to get your message blocked. But added to other spam criterias (ex : if you message has already been identified as a mailing list), that may give you some bad surprises.