An SMTP server configured in such a way that it allows anyone on the Internet to send email through it.
An SMTP server configured in such a way that it allows anyone on the Internet to send email through it, not just mail destined for or originating from known users. This is not a recommended configuration because it can be exploited by spammers. Servers with open relays are routinely blocked.
To get more information please check out our Email Infrastructure Guide.