The usual failure
Every send is 'the send'. You cannot tell if revenue moved because you mailed, or because it was going to move anyway.
What to make true
A group you do not mail. Compare. If you will not hold out, do not pretend the campaign proved itself.
Worked example
Keep it this plain. A team uses holdout / control as if buying the software installed the meaning. It did not. Every send is 'the send'. You cannot tell if revenue moved because you mailed, or because it was going to move anyway. The move is not a new integration. A group you do not mail. Compare. If you will not hold out, do not pretend the campaign proved itself.