When you reply to a ticket (either to Caller or Responsible), it is sent from the e-mail address you have specified in the ‘Send from address’ field available under the tab Other Settings in the Folder HelpDesk Settings. Could the problem be that you have not specified an address there?
If you have written in the address you want in the Other Settings field, please check the following:
Have you configured a common email account for outgoing e-mails from the helpdesk? If a common account is setup successfully, any correspondence from the Folder HelpDesk (eg. from the ticket form) will be routed through the common account.
Is the staff replying to caller from the emails directly, (from the e-mails in the sent/received folders)? Replying to e-mails from sent/received would not be routed through the common account. Instead , they should use the ’email to caller’ button in the ticket form.
All helpdesk staff needs to have ‘send behalf of’ permission set under the common mailbox account in Active directory.