Resolution:To resolve this issue check the checkbox next to "My Server Requires Authentication".
Open Outlook > Click Tools > E-Mail Accounts > Select the email account > Click Change > Click More Settings > Click the Outgoing Server Tab > Check the checkbox "My outgoing server (SMTP) requires authentication" > Click OK > Next > Finish
By authenticating with the server, it knows you are authorised to send mail outside of our network and will instead forward the mail to the correct mail server for the domain.