You can change the subject line of your newsletter under the 'Send From' tab on the design page. There are three options for the subject line...
1. You can create a custom subject line that will be the same for all of your subscribers and/or combine the custom subject line with merge tags.
2. You can use the title of a boosted article. If you have multiple boosted articles, the title will be chosen from those or you can super boost an article to ensure it is the same for all of your subscribers.
3. You can use the AI generated subject line. The AI generated subject line will use the title of the top recommended article for an individual subscriber as the subject line.