Sending emails from a VPS server must be done through an SMTP server installed directly on your VPS machine.
You cannot send mails through smtp.simply.com.
On Linux you can send through f.x Sendmail or Postfix, and on Windows you can use the SMTP Service in IIS.
Be sure to send the mail from a valid domain, both in "From" and "Return Path".
Article from the support category: VPS
