When you add a user that has used that email address to sign via Adobe Sign (at any point in time) you may see this error: "Error Message: You cannot add that email address to your account"
If you have a paid Adobe Sign subscription, please reach out to support and provide the email address of the person(s) you are trying to add - they can assist you further. Here are instructions on how to open a support ticket - https://spark.adobe.com/page/DOwypsNOXKAu7/
Then this must has something to do with the users has signed via Adobe Sign earlier.
But how can we test Adobe Sign, when half of our users have this trouble? What is the point of a trial if we need to pay for a subscription to get this fixed anyway? There is no logic in that.
And also as a Adobe reseller, if a customer also has this problem, we have to say to the customer that "you need to purchase the product, so you can test it." I don't think our customers want to do it that way.