The subject of your email is perhaps the most important few words in the entire email. It is the first impression, it is your tagline, it is the reason the recipient will, or will not open it. The purpose of the subject line is to get the person reading to say three simple words: "Tell me more."
If you think about it, an email's subject is much like a company tagline. Often, entrepreneurs will ask me about a tagline or more often, they will ask me to choose between two options. The first one is usually descriptive, explaining what the product is, or does. The second one is more inspiring and has one goal and one goal only, to get me to say "That sounds interesting, tell me more."
Your tagline is not meant to tell me everything, just like a subject line is not meant to save me the time of opening the email. On the contrary, the subject is there to get me to click on that email as opposed to that Delete button.
With that in mind, here are some mistakes to avoid when writing a subject of an email:
1. Writing the whole email in the subject line.
I am not going to address what type of person does this, but I will say that it is up there with the most annoying email practices. Writing out the entire email in the subject and leaving the body of the email empty is... Just don't.
For starters, subject lines are meant to be kept short, and as a result, on mobile, where we all read our email, a long subject line is not displayed properly. It is hard to read and just not the way email was intended to be used.
I am not even going to mention adding a link in your subject, which makes it unclickable. If I wanted unclickable links, I wouldn't be in my inbox, I would be in Instagram.
2. Clickbait: Don't send it.
We have enough clickbait around the internet, we do not need it in our inboxes as well. If your subject is so clearly written in a way that it is there for the sole purpose of getting me to click and not actually providing me some context, well, you will achieve the opposite result. You wanted me to click? The only thing I am clicking is Delete. Leave the clickbait to YouTubers and focus on telling me why I should care enough to read the actual email.
3. You have a few words, don't make them meaningless.
"Hey", "Dear Hillel", "So..."
I mean, really? You had one job, to get me to click on that email by telling me what it is about and why I should care. That is what you wasted that opportunity on? A meaningless waste of characters? Think about the recipient and imagine that your email is one of ten thousand that the person has to go through. If that were the case, would you still have written "Hey" as a subject line? Probably not.
How to Write a Solid Subject Line
If you are sending an email introduction between two people, consider writing the subject "Intro David (Google) / Jennifer (Facebook)". It is informative, it is straight forward, and it gets the job done.
If you are asking to meet someone, maybe use something like "A 30 minute cup of coffee?" It says explicitly what your goal is with sending the email, adds a personal touch, and has the recipient asking himself "Why? What? When? Who?" Most people, at least the ones I know, when someone wants to meet them, are curious why, and would say to themselves "Tell me more" right before opening that email.
The point is, use the Subject line to summarize what the person can expect when reading the email, and try to do it in a personal manner, while not trying to manipulate anyone with some well known internet tricks.
Of course, the email itself needs to follow suit, but how to write an effective email is a whole topic in and of itself.