开放转发（open relay）是一项SMTP电邮服务器的设置，使得互联网的所有用户都可以透过该服务器转发电邮。

1990年代前，一般寄送电邮的方法是将电邮由一台电脑传到另一台，这样的方法通常不太有效。因此，有了电邮转发（mail relay）的概念。电邮转发可以设置黑名单或白名单，当然也可容许任何用户透过它转发邮件，即开放转发。开放转发是UNIX上的电邮服务器的缺省设置。由于早期的互联网多是教育和政府机构使用，没有甚么商业机构，所以垃圾邮件并不是甚么严重问题。

现时，随着技术进步，各种网络协议已经有了将消息一个传一个的能力，转发的功能已无用武之地。

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.