How do ROI-related words impact cold email subject lines?
If people are interested in cold emails promising a discount or opportunity, then subject lines that contain words related to other business benefits should be as successful, right?
These are the words we’ve decided to analyze in our research next. Words like improve, increase, scale and others seemed really promising. Yet, our data scientists surprised me again!

All in all, using ROI language in subject line for cold email can significantly damage your open and reply rates, decreasing them by up to 34% and 66.5% respectively. I see two possible reasons for that: Either people are skeptical about any direct benefits offered in a cold email or subject lines like those are overused to the point that they lost their impact.
The words grow and boost seem to be the only exceptions, driving results that are slightly above the average. I would highly recommend running a few A/B tests with them to see if you can get better results.
Bottom line, don’t get carried away with ROI language in your cold emails subject lines – such words can significantly harm your open and reply rates (with just a few exceptions).
Effective subject lines to steal:
- Let’s Grow Together
- {{FirstName}} – {{Pain Point}}
- Low-cost {{Pain Point}} for {{Company}}