Your company name
When you use your company name in your subject line, you are immediately putting yourself first; this should be the customer. You do not know if they have even heard of your business before, let alone if they are interested in what you have to offer. Personalise it by using "You" or their first name. Emails with a personalised subject line result in a 22.2% increased open rate. Only use your company name when the receiver knows you, as in a follow-up email for example.
There are many words that are known for triggering recipients' SPAM filters. Things like "Click Here" or "Act Now" are a spam filter's favourite catch, but nothing beats the word "Free". Sometimes known as the ultimate SPAM word, variations of this such as "Free eBook" and "Free Sample" will push your email into the dark abyss of the junk folder.
Don't make it too long
Gmail allows for subject lines to be 80 characters; however this does not apply to mobile. According to Litmus, 54% of emails are opened on mobile. Keep your subject line short to reduce the chances of it being cut off. It also helps to keep attention on the most important part of the subject.
DON'T USE ALL CAPS
This is a massive put off to recipients, as it looks spammy and outdated, and is quite frankly just annoying. This is the same as too many emojis. If you are going to use one or two make sure to check that your email subject line is being read correctly and people aren't going to see the blank square instead. While we are on the subject of annoying, stay away from using lots of exclamation marks!!!!!!
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Don't be too vague
Mystery and surprises are very exciting but not when it comes to email marketing. Make sure your subject line gives the reader an idea of the main topic of the email. This is more likely to get them to open than a generic subject which doesn't relate to the content within the email. People do not like to be lured into something only to be given something else, don't be vague and reduce the chances of this happening.
Subject lines seem like they should be so simple but they are in fact the most important part of email marketing. 35% of people open an email based on the subject line, so stay away from these mistakes to maximise their effectiveness.