MUA
邮件阅读或发送程序,如 outlook, 在邮件系统中用户只与 MUA 打交道,MUA将邮件系统的复杂性与用户隔离开。
Mail Router
程序,从用户处接收邮件并决定其目的地址以及如何到达目的地。比如根据接收者的地址不同,电子邮件可能通过TCP/IP网络发送,或者通过UUCP或FAX发送。邮件路由使用接收者地址及其内部的配置信息来选择一个最好的MTA,然后将邮信件转给此MTA。
MTA(Mail Transport agent)
一个专用程序,其作用类似于邮局,用于在两个机器之间发送邮件。通常,一个机器上只有一个MTA。sendmail程序就是一个MTA,此外还有其他MTA,如MMDF,Smail 3.x, qmail以及zmailer等。
MTA能够理解特定网络的EMAIL协议并通过网络传输信件,如UUCP可通过UUCP连接发送信件,但无法处理SMTP信件。
MDA(Mail Delivery Agent:投递代理)
sendmail自己并不完成最终的邮件发送,它要调作其他的程序来完成最后的投递服务。在SVR4系统中一般是/bin/mail.
信封(envelope)和内容(content)
一个email消息由两部分组成:信封(envelope)和内容(content)。
信封告诉SMTP代理(sendmail或者postfix)如何投递消息。
内容又包括信头(Header)和 消息内容。Header 和消息内容之间用一个空行分隔。内容包括能被人们阅读的消息本身和一些标题(header)(比如消息subject),而一些内容中的header可能和envelope中的重复(比如"To"地址),这些重复的header用来显示时候使用,而envelope中的则是用来投递使用。(这也是为什么你会收到"To"地址不是你的垃圾邮件)
Envelope sender (ES), Envelope recipient (ER), Header sender (HS), Header recipient (HR)
Envelope sender (ES) -- The address provided by the MAIL FROM: SMTP command. Mail transport agents send delivery errors to this address, and often check for the existence of the hostname portion of this email address in DNS. For this reason, if you are rewriting/masquerading the HS, it may be a good idea to masquerade the ES.
Envelope recipient (ER) -- The address provided by the RCPT TO: SMTP command. It is the final destination address of the email. This may differ from the HR if the address is rewritten using the user database, aliases database, or virtusertable.
Header sender (HS) -- The From: address in the message header. This address is frequently masqueraded/rewritten from user@host.domain.com to user@domain.com, especially when host.domain.com is not configured to handle mail directly.
Header recipient (HR) -- The To: address in the message header. This is the recipient address specified by the author of the message. It is left intact unless you use the allmasquerade feature or domaintable.
$ telnet smtp.mydomain.com 25 Trying smtp.mydomain.com... Connected to smtp.mydomain.com. Escape character is '^]'. 220 smtp.mydomain.com ESMTP mail from: mailfrom@mydomain.com 250 ok rcpt to: jiangxin 250 ok data 354 go ahead Date: Tue, 1 Mar 2008 16:10:44 +0800 From: Header-From <header-from@mydomain.com> To: Header-to@mydomain.com Subject: smtp test Hi, This mail contains only a test message. Ignore it. Sincerely yours, Johnson .
Copyright © 2006 WorldHello 开放文档之源 计划 |