First, you should always protect against your machine — the one serving your Web content to millions of users — from being used as a mail relay.

首先，应当始终防止您的机器(为数百万个用户提供Web内容的机器)成为一个邮件中转站。

youdao

Back then many companies ran a free, open-source e-mail relay technology but ended up spending more than they thought they would making the stuff work.

FORBES: You Get What You Pay For

Some use "mail relay, " borrowing someone else's domain to send e-mail.

FORBES: Spam Warfare

Back in the early 80s a man by the name of Geoff Goodfellow had an idea: to relay electronic mail from Arpanet to his alphanumeric pager.

ENGADGET: Geoff Goodfellow, early inventor of wireless email, profiled