The line right after your subject line is doing half the work, and most people leave it blank
Preview text, the snippet that shows after the subject line in the inbox, is prime real estate that most senders fill with whatever text happens to appear first in the email. Usually that's "view this in your browser" or nothing at all.
Write it on purpose. It should add information the subject line didn't have room for, not repeat it. If your subject is "The mistake that cost us $10k," the preview text can answer the question the subject raised: "It wasn't the ad spend." Now the two lines work together instead of one going to waste.
Check your last few emails. If the preview text is a boilerplate link or blank, you've been giving away free space in the inbox.