為什麼信件標頭中的Received有很多行?

這是因為在郵件中繼(mailrelaying)的過程中,每一次回應就會做一次紀錄。

Received: from cp4.oz.nthu.edu.tw (cp4.oz.nthu.edu.tw [140.114.63.144])
	by oz.nthu.edu.tw (Postfix) with ESMTP id 13B5D3FA5B;
	Mon, 20 Jul 2009 09:46:57 +0800 (CST)
Received: from smtp63-1.oz.nthu.edu.tw [(140.114.63.55)] by cp4.oz.nthu.edu.tw
	(envelope-from <sec-request@list.net.nthu.edu.tw>)
	(NTHUCCC AntiSPAM Mail Server)
	with ESMTP id 844053685; Mon, 20 Jul 2009 09:46:15 +0800
Received: from smtp2.oz.nthu.edu.tw (smtp2.oz.nthu.edu.tw [140.114.63.52])
	by smtp63-1.oz.nthu.edu.tw (Postfix) with ESMTP id C27D5423B8;
	Mon, 20 Jul 2009 09:44:05 +0800 (CST)
Received: from list.net.nthu.edu.tw (list.net.nthu.edu.tw [140.114.63.40])
	by smtp2.oz.nthu.edu.tw (Postfix) with ESMTP id 2B99042759;
	Mon, 20 Jul 2009 09:44:05 +0800 (CST)
Received: by list.net.nthu.edu.tw (Postfix, from userid 95)
	id 69FC05527E; Mon, 20 Jul 2009 09:44:00 +0800 (CST)

舉上述例子說明,此封信的收件人的網域為oz.nthu.edu.tw ,所以從寄件人的信箱寄出郵件後,而伺服器list.net.nthu.edu.tw收到後查找網域將信件轉發到伺服器smtp2.oz.nthu.edu.tw,再查找網域將信件轉發到伺服器smtp63-1.oz.nthu.edu.tw,最後查找到cp4.oz.nthu.edu.tw,然後將信寄發出去;這些動件就是郵件中繼(mailrelaying)。