The option to override the 'Send From' user allows you to Override all setting for Ownership or Market From, as set by default for the 1 email send job in particular.

By default you want this setting to remain 'Market From', which will keep the original user as the Sender for each individual recipient.

If you do not have any options in your 'Send From' drop down, that means you have not set up and Users or Roles to be used in conjunction with this tool. Reach out to our team directly to set up this field or if you have any questions - [email protected]