When you reply to a ticket (either to Caller or Responsible), it is sent from the e-mail address specified in the ‘Send from address’ field available under the tab Other Settings in the Folder HelpDesk Settings.
Exchange must be configured to allow each staff member to send messages from Folder HelpDesk from an e-mail address other than her/his personal one.
If no address is specified, the e-mails are sent from the senders personal e-mail account.