什么是 "sender-SMTP connection"？

zhoulzu

在fetchmail的manpage里有这么一段：
[code:1]The ETRN mode allows you to ask a compliant ESMTP server (such as BSD sendmail at release 8.8.0 or higher) to immediately open a sender-SMTP connection to your client machine and begin forwarding any items addressed to your client machine in the server's queue of undelivered mail. [/code:1]
其中所说的“sender-SMTP connection”是什么意思？

谢谢！