邮件服务sendmail和dovecot

用于传递信息的标准协议：
  SMTP:
    tcp 25

用于接收的协议：
  POP3: Post Office Protocol
    tcp 110
  IMAP4: Internet Mail Access Protocol
    tcp 143
  MIME: Multipurpose Internet Mail Extensions
    base 64

MUA: Mail User Agent(邮件用户代理)

    web mail
    outlook, oe, foxmail, thunderbird, evolution
    mutt
    telnet Server 25
MTA: Mail Transfer Agent(邮件传输代理)

    Exchange
    Lotus Notes Domino
    Sendmail, Postfix, Qmail，Exim
MDA：Mail Delivery Agent(邮件投递代理)

    Mail Delivery Agent
MAA: Mail Access Agent(邮件访问代理)

    POP3, IMAP4
    courrier-imap, dovecot
MSA:提交

echo "HOW ARE YOU?" | mail -s 'Hello' terra

sendmail, sendmail-cf, sendmail-doc, m4

sendmail.cf: sendmail.mc
access.db: access
local-host-names
/etc/aliases.db: /etc/aliases

邮件别名：
  [email protected] --> [email protected]
  /etc/aliases
    test1:  test2

接收邮件：
 dovecot
  /etc/dovecot.conf

USER
PASS
LIST