Q:

Sharepoint doesn't send email to "Current Item:Created By"

My sharepoint workflow seems to only send email to "Current Item:(Name of user)":

instead of the list variable "Created By" as in "Current Item:Created By".

Does anyone know why this happens? 
The error sent to my smtp server email is:

Delivery has failed to these recipients or groups:
  Created By
  The format of the email address isn't correct. A correct address looks like this: someone@example.com. Please check the recipient's email address and try to resend the message.

But "Created By" is a variable. It shouldn't be in an email format. Also, last I checked, this variable has a name inside. It's not empty.

A:

In the picture below I have used the field 'Assigned To', you have used 'Created By', but in order for Sharepoint to be able to send an email, it does need an email address. 

If I substitute my Assigned To person/group field for the Created By field you'll see you can define the field to be used as an email address - select this one in your workflow, republish and save.

