Where to set the subject line
Open the email campaign and type your subject line in the subject field at the top of the editor. The same field is used for every recipient unless you enable A/B testing (see below).
Preview text
Below the subject field you can enter preview text. This is the short snippet that appears after the subject line in most email clients before the reader opens the message. Keep it under 100 characters. If you leave it blank, the email client will pull the first visible text from the body instead, which is often not what you want.
What makes a subject line work
Be specific. "Can you help by Friday?" outperforms "Important update." Readers decide in under two seconds.
Match the ask. If the email asks for $25, the subject line should not tease a $1,000 match. Mismatches hurt click-through even when opens are high.
Shorter is safer. Most mobile email clients cut off subject lines after around 40 characters. Front-load the key words.
Personalization tokens work if your data is clean. Adding the recipient's first name or city can lift opens, but a broken token ("Hi ,") does the opposite.
Avoid spam trigger phrases. Words like "FREE," "ACT NOW," and strings of exclamation points increase the chance your email lands in a spam folder or promotions tab.
A/B testing subject lines
You can test two or more subject lines against each other using the A/B testing feature. When A/B testing is enabled, you set a percentage of your audience that receives each variant, and you set a wait time before the winner is determined. After the wait period the campaign automatically sends the winning subject line to the rest of your list based on open rate.
Each variant has its own subject line and preview text field.
Opens are an estimate
Open tracking works by embedding a small invisible image in the email. When that image loads, the send is counted as opened. Apple Mail Privacy Protection and similar features cause some opens to be counted even when the recipient did not actually read the message. Treat open rate as a relative signal for comparing subject lines rather than an exact measure of engagement.