Assuming an email gets delivered AND the contact sees it, the first thing impacting the open rate is the subject line. Here's a quick checklist of things to consider as you build effective subject lines.
Length of Subject Line
Keep the subject line short – 7 words and 41 characters are optimal.
Use emojis to help your subject line stand out.
Make the subject line unique to the recipient.
Take the time to A/B test your subject lines.
Find the balance between trendy and personalized.
Word Choice in Subject Line
Power Words
Industry Keywords
Security
Simplicity
Engaging
Establish Urgency
Act Now
Limited-time offer
Instant savings
Save today
Deadline approaching
Limited number available
Time sensitive
Don’t miss this
Hurry
Create Curiosity [1]
Ask a curiosity-inducing question
Start a sequence of events, but don’t finish (e.g. an unfinished story)
Violate expectations
Imply that you have information they don’t
Imply that they used to know something that they’ve since forgotten
[1] Loewenstein, G. (1994). The psychology of curiosity: A review and reinterpretation. Psychological bulletin, 116(1), 75.