了解email发送过程
参考链接中说的很详细了，这里再重复。
Simple Mail Transfer Protocol (SMTP)
Post Office Protocol (POP3)
Mail User Agent (MUA)，常用的如Outlook、foxmail等软件。
Mail Transfer Agent (MTA)，常用的有Sendmail、Postfix等软件。
Mail Delivery Agent(MDA)，有的MTA同时具有MDA的功能，如Sendmail、qmail等。也有独立程序如procmail。
|
Part |
What It Does |
Examples |
|
Mail User Agent (MUA) |
The MUA is the program which the user uses to read and send e-mail. It reads incoming messages that have been delivered to the user's mailbox, and passes outgoing messages to an MTA for sending. |
elm, pine, mutt |
|
Mail Transfer Agent (MTA) |
The MTA basicly acts as a "mail router". It accepts a message passed to it by either an MUA or another MTA, decides based upon the message header which delivery method it should use, and then passes the message to the appropriate MDA for that delivery method. |
sendmail, postfix, qmail |
|
Mail Delivery Agent (MDA) |
The MDA accepts a piece of mail from an MTA and performs the actual delivery. |
mail.local, procmail |
参考：
http://tech.ccidnet.com/art/3089/20071017/1245123_1.html
http://linux.vbird.org/linux_server/0380sendmail.php
http://support.kavi.com/khelp/kmlm/user_help/html/how_email_works.html
http://en.wikipedia.org/wiki/Email
http://wiki.list.org/pages/viewpage.action?pageId=4030516
http://www.feep.net/sendmail/tutorial/intro/MUA-MTA-MDA.html
http://en.kioskea.net/contents/courrier-electronique/fonctionnement-mta-mua.php3
