About A/B Split Testing
You can create different versions of your message, test them on small random subsets of your mailing list, and then compare the results to see which message is most effective.
This is called split testing or A/B splits and the subsets of your mailing list that you isolate for testing are called distribution groups.
This kind of testing eliminates any demographic or action-based bias that could alter your results.
Once you determine which message gives the best results, you can send it to the rest of your list.