Email subject lines – to symbol or not to symbol that is the question?

When conducting a recent offer mailing for a client – we ran with a rather ‘safe’ subject line.  I say safe – in that, it wasn’t very specific to the mailing.  For example – the offer we were running was a 24 hour only offer - all company credit E-mailreports £1 for 1 day.  However, best practice preaches that the use of numbers and symbols cause issues with deliverability.  And therefore, we always steer clear of using them.  So instead of running with a symbol loaded subject, we ran with a headline of ‘ 24 hours of winter madness’.

So, 1) no mention of company credit reports,  and, 2) no mention of the actual offer.  However, we hoped that we’d intrigue people into opening to find out what it was about.

The open rates pretty good (avg 25%) and in some groups, the click throughs were very respectable.  However, with such a great offer, we were expecting better things.

Following that mailing, we tuned into a webinar, ‘The five best ways to optimise email response’

by Dr Flint McGlaughlin.

Many of the lessons within the webinar were as expected, however, it did provoke thinking.  In particular, when they were going through a series of subject lines and looking at how the campaigns could be improved – one recommendation was to change the subject line of the mailing from ‘Thank you for Making us your Florist of Choice, to ‘15% offer – Our way of saying thank you!’.


Now of course, any marketer utilising basic best practice techniques is going to be aghast at such a recommendation – let’s see, how many symbols in the subject line – errr…% - and ! =3!

Now of course, from a marketing message – I totally agree.  Communicating very clearly what you are offering right up front is key to successful marketing messaging.  However, what about the spam filters and deliverability – what’s the point of going to all the effort of creating the piece, if it isn’t going to make it through to the recipient.

So we decided to query this point with Dr Flint McGlaughlin – and his response is detailed below.


How much validity is there to the conventional wisdom that, in the Subject line of an offer email message, numbers, certain symbols (especially £/€/$, %, and !) and “SPAM words” such as “Free” and “discount” will cause a dramatic reduction in deliverability, and consequently effectiveness?


In the case of the particular company and study referred to on Slide 22, that was precisely one of the questions we set out to answer.  What you couldn’t see in the context of Dr. McGlaughlin’s Email Summit presentation is that this particular 2-treatment comparative vignette was just a tiny part of a much larger and broader study intended to test that specific widely-accepted presumption along with a host of others to see how valid they remained through the evolution of regulatory and ESP-technical filter changes since the time they were first introduced and anecdotally adopted; around 2003-2005.  This was important because we know from our foundational Offer/Response-Optimisation principles of “clarity trumps persuasion” and “specificity converts”, that the clearer and more specific subject line—i.e., the one with the “15% Off…” copy—should convert better.

What we found was that there was, in fact, a small but significant difference in deliverability—interestingly, it was more pronounced among the smaller ESP’s.  In addition, as we had predicted based on the “eme” heuristic, the Open Rate actually declined (…by more than 25%).

In the end, though, the central research question was “Which email subject line will result in the greatest projected net revenue?”   As revealed in Dr. McGlaughlin’s presentation,  despite the slight dip in Delivery rate, and the (what would otherwise have been alarming) drop in Open rate, the Clickthrough Rate (CTR) to the landing page was 60.3% higher.  What he may not have mentioned is that, in direct answer to the research question, the Treatment subject line yielded a 56% increase in projected net revenue vs. the Control.

So, while it appears there is still at least some validity to the commonly held belief that special characters in the email Subject line reduces deliverability, our research—this test two others conducted in different products and industries—suggests that when they serve to do so, these negative factors are dwarfed by the power of clarity.

Interesting eh – I’ll certainly be split testing the subject line in our next offer against this view – and will report back results.

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Michelle Carvill is owner and Marketing Director at Carvill Creative - a graphic design and marketing services agency based in Maidenhead, Berkshire.  The agency covers all aspects of graphic design and marketing - covering social media marketing and website planning and website design.