Don't manually put RE in your email subject lines as email services consider the tactic underhanded and may label you a high-risk sender, mark your emails as spam, or display a brand-damaging alert to your recipients about how it's not actually a response to a message they've sent.
One summer, when I was 12, my parents shipped me from our home in Ohio to the deep south of Thomasville GA - to spend four weeks with my grandparents.
Other than a particularly nasty fire-ant incident, I don't remember much of the trip. What I remember is coming home and my friends good-naturedly mocking me for saying "Y'all" and speaking the with the slightest bit of a southern twang.
Humans are incredibly good at picking out the slightest tic's in language and while we don't generally think of it - there's a massive difference in how we tend to communicate with those we know (personal) and those we don't (professional).
Email marketers started prepending the subject lines of cold emails, newsletters, and other commercial messages with 'RE' in an attempt to short-circuit their recipient's defenses and catapult themselves directly into "personal" territory.
When your sweaty hand clicks the 'Send' button you don't really care about the technical details of what's happening, you just want results.
And the first of those results you're likely to see is "Opens" - the percentage of people who when they saw your subject line thought: "this looks like something I should pay attention to" and opened the email to take a closer look.
And if you ran an A/B test on two subject lines:
GROUP A: Have you downloaded your whitepaper yet? GROUP B: RE: Have you downloaded your whitepaper yet?
All else being equal, you're probably going to get more opens with "B", because you're tricking people into thinking that it's something they're in the process of working on and communicating with someone already.
What won't be captured in that test is what happens next: people getting angry they've been tricked, people unsubscribing, reporting you for spam, and potentially cursing your name on Twitter, etc.
Odds are you're not being paid to get email opens, you're being paid to achieve a real business objective: sales, leads, or at the very least to not actively tarnish your brand.
The first thing to understand is that it's incredibly easy for an Email Service to detect a fake "RE" in a subject line as the email headers (all of the information sent along with your email that describes what server sent it, who it is for, the unique message id and other technical and diagnostic information that you don't typically need to see) will list the email ID that it's in response to if it's real.
It's not ambiguous in the least and while it's still a minority of email services that are outwardly proclaiming it, many are beginning to use it as a weight in their spam and other filtering calculations.
Fake "RE"s succeed in boosting open rates because they stand out in an inbox. Adopt other methods of standing out and making a rapid connection with your recipients like:
To test your subject lines's for other issues and get more suggestions like these, try our Subject Line Tester