The acknowledgement email is only available to be sent to the email address the constituent entered on their form. If they entered an incorrect email address, then you would need to manually send them an acknowledgement. You could send them an email through Email>Messages, building a list and a message and possibly a template), or send them an email using your organization's email address, through Outlook for example.

You may also consider generating an acknowledgement letter from the gift record within The Raiser's Edge.

Can An Acknowledgement Email Be Re-Sent From NetCommunity?