Use sharepoint list column as email address in workflow

I apologize for the lack of my knowledge about Nintex but have not found a way to do the following yet:

BACKGROUND:

I have a sharepoint list; one column (which is a lookup column) contains email addresses of doctors.other column contain data about status of submission of documentation.

GOAL:

I would like to send a notification to individual doctors on the list according to a condition I set (based on the value of another column on the sharepoint list)

I know how to create a notification, I know how to set the condition BUT....

PROBLEM:

I can't find a way to use a specific email off the list as a variable in the "TO:" field of the workflow notification;

 I am able to put an email address that authenticates to my own company's Active Directory but the individual Doctors on my list are all third pafty doctors.

Could you please provide me with the pointer to the right path/solution (or where I should pose this question if I am posting on the wrong spot).

Thanks,

Christopher