The KEY focus when you’re sending a cold email is to ensure that your subject line relates immediately to the specific thing you’ll do to help their business. It’s okay to keep the subject line generic instead of overpromising something that may seem too far-fetched. Try both out and see what works better for you. Personally, we've found that saying something more actionable/tangible nets a higher response rate. Make your proposal something quantifiable and relevant. A generic message will decrease your response rate.
