现象：
    telnet
    mail.aaaaaaaaaaaaaa.com 25
    Trying ddd.ddd.ddd.ddd…
    Connected to mail.aaaaaaaaaaaaaa.com.
    Escape character is '^]'.
    220 ****0**********************************
    ehlo aaaaaaaaaaaaaa.com
    502 Error: command not implemented
    helo aaaaaaaaaaaaaa.com
    250 mail01.aaaaaaaaaaaaaa.com
    ehlo aaaaaaaaaaaaaa.com
    502 Error: command not implemented
    连接后出现
    220 ****0**********************************
    并且不支持ESMTP
    解决办法：
    no fixup protocol smtp 25
    If you're getting the *** banner, and EHLO not recognised and you have a PIX
    in the way, it's fixup, and there's nothing else you can do. To give ammo if
    you need it, Cisco deprecated fixup in v7 of the PIX OS, due to it not
    understanding anything beyond RFC 821.
    From their own docs:
    The fixup protocol smtp command enables the Mail Guard feature. This restricts
    mail servers to receiving the seven minimal commands defined in RFC 821,
    section 4.5.1 （HELO, MAIL, RCPT, DATA, RSET, NOOP, and QUIT）。 All other
    commands are rejected.
    Microsoft Exchange server does not strictly comply with RFC 821 section 4.5.1,
    using extended SMTP commands such as EHLO. PIX Firewall will convert any such
    commands into NOOP commands, which as specified by the RFC, forces SMTP
    servers to fall back to using minimal SMTP commands only. This may cause
    Microsoft Outlook clients and Exchange servers to function unpredictably when
    their connection passes through PIX Firewall.