1.邮件传输协议
SMTP:Simple Mail Transfer Protocol
POP3:Post Office Protocol
IMAP4:Internet Mail Access Protocol
2.smtpd监听(25/tcp)
3.SASL:Simple Authentication Secure Layer
cyrus-sasl
courier-authlib
4.MDA:邮件投递代理
procmail,maildrop
5.MUA:邮件用户代理
mutt(文本界面),mail,thunderbird,evolution
6.MTA:邮件传输代理
sendmail,qmai,postfix,exim
7.MRA:邮件检索代理(pop3,imap4)
cyrus-imap
dovecot