Emails sent from my server need authentication

My server sends out emails when clients place orders and we send invoices through our server. Many clients do not receive the emails because they are not authenticated. Here is the message from my host as to why they arent authenticated: Your emails being sent from the server are showing as unable to be authenticated as having been sent from [login to view URL] because of the following: Return-Path: <hdshow@[login to view URL]> This means that the script isn't setup for SMTP authentication which means it's not configure to login to the email account info@[login to view URL] with the appropriate username and password prior to sending the email. You'll have to modify your script to use SMTP authentication in order to resolve the issues. So that is what I need done for at least 3 pages that send emails.