What is Reply To address in Send Mail action?

It is the email address that can be replied to by the leads. You have the following options to choose from:

• Enter a From address - Lets you enter an email address manually
• Helpdesk support email ID - Use the email address set up under Settings > Configuration > Email Settings as Helpdesk Email
• Assigned To: (Users) Primary Email - Use the email address of the user to whom the lead is assigned
• Last Modified By: (Users) Primary Email - Use the email address of the user who last modified the lead record (check the Last Modified By field in the lead record)
• Assigned To : (Users) Reports To - This is the email address of the user whom the case assignee reports to

To learn more about the Send Mail action, click here.