The right subject line is key to get your emails past spam filters and tempt recipients to open the message. Depending on what platform you use to send emails, you may have access to tools to help improve your email subject lines. But if your email marketing software lacks those tools or you’d like to improve emails you send through webmail or a desktop client, there are online tools you can use to improve your subject lines.

Start by trying these free options:

Most email subject line testing tools look for word and character counts, words associated with spam messages, sentence format, and whether it contains active or urgent words. Some tools may also display how subject lines and email previews look in different browsers, email clients, and on mobile devices.