No, you don't need a mail client or server on the ePO server: just the details of the SMTP server you want to use to send mail. With regard to the account - that will depend on the SMTP server that you're using: some will refuse to accept mail from an unknown account. It's probably easier to create an account just for rPO notifications and use that, I would think.
Thanks Joe, your answer is really helpfull to me!
With kind regards