An open mail relay is an SMTP server configured in such a way that it allows anyone on the Internet to send e-mail through it, not just mail destined to or originating from known users. This used to be the default configuration in many mail servers; indeed, it was the way the Internet was initially set up, but open mail relays have become unpopular because of their exploitation by spammers and worms. Many relays were closed, or were placed on blacklists by other servers.
SMTP Relay lets you send email messages through your email account using your existing email service. For example, you can continue to use Microsoft Outlook© to compose, receive, and send email messages, but the actual email messages are processed through our SMTP relaying services. This lets you bypass ISP restrictions on your outbound email messages, and allows you to use your professional looking “firstname.lastname@example.org” email address from any service you wish.