a mail server that allows anyone from anywhere on the Internet to send email through it to a remote recipient
a mail transport agent (MTA) that is configured to forward mail between senders and recipients who are not a part of the MTA's operational domain
an e-mail server that accepts and routes mail from and to users not associated with the server's domain
a server that will accept mail from any user on the Internet that is addressed to any other user on the Internet
a SMTP mail server that processes mail where neither the sender or the recipient is a local user
a SMTP server configured to allow anyone on the internet to relay (send) a message using it
An open mail relay is an SMTP (e-mail) server configured in such a way that it allows anyone on the Internet to relay (i.e. send) e-mail through it.