It’s all about the headline. The headline will make or break your email campaign. There is nothing more important.
Bottom line: write subject lines that get opened. Understand what captures attention and motivates someone to read more.
Write conversationally, as though you were speaking to a friend instead of writing business correspondence. Write your subject line in that friendly tone, much as you would if you were writing to a friend. One way to convey this is to write in sentence case (only the first word is capitalized.)
Subject lines that generate an emotional response are far more likely to be opened than logical subject lines. Make the reader curious, feel urgency, or looking for an answer.
Use concrete details because details help capture attention. Details in your subject line grab your reader’s attention and create an emotional response that drives opens. Specificity also creates a sense of credibility.
Length is secondary
Character limit rules for subject lines are usually driven by character display limits in email in boxes rather than by customer behavior. Being emotional and specific is far more important than fitting into a set number of characters.
Your best guide to the idea length is the preferences of your specific audience. Some audiences like short subject lines. Others prefer longer ones. Try long and short subject lines and see what works best.