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421 Too many concurrent SMTP connections; please try again later

Discussion in 'E-mail Discussions' started by eurorocco, Jul 3, 2014.

  1. eurorocco

    eurorocco Well-Known Member

    Jun 23, 2003
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    421 Too many concurrent SMTP connections; please try again later

    In /etc/exim.conf
    I have smtp_accept_max = 100
    which always was fine for customer traffic.

    Today I find servers are under a distributed attack. From many many different IPs some hacker is hogging SMTP connections. From each IP some thirty or forty SMTP connections are opened. I blocked a dozen IPs and a new dozen IPs come up to hog SMTP connections. I blocked the dozen and a new dozen comes up.

    I'm looking for a way to limit connections per IP.

    Perhaps I will have to write a script to automatically block IPs that take more than 5 connections and are not authenticating at all. I hope not.

    I did see other threads with subject "421 Too many concurrent SMTP connections; please try again later", but they were too old and the forum didn't alow me to post on them because they were too old.

    Any suggestions?

    Thanks a lot for your help!

  2. cPanelMichael

    cPanelMichael Forums Analyst
    Staff Member

    Apr 11, 2011
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    Root Administrator

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