SOLVED: 4.2.1 Unable to connect.” Attempted failover to alternate host…

If you see the following error in your Exchange 2003, 2007, or 2010 queue, there are three things you should do to troubleshoot the problem:

451 4.4.0 Primary target IP address responded with: “421 4.2.1 Unable to connect.” Attempted failover to alternate host, but that did not succeed. Either there are no alternate hosts, or delivery failed to all alternate hosts.

Below are the three items you need to check:

      1. Test the connection by sending an email through command line.  This is MUCH easier than it soundsOpen a CMD command prompt on any PC or ServerType telnet <DestinationEmailServerName>25 and press enter.

        Type helo and enter.

        Type mail from: and enter.

        Type rcpt to: and enter.

        Type data and enter.

        Type subject: test and enter.

        Type test and enter.

        Type . and enter to quit the session

        If you do not see any errors, this proves the problem is with your server and not with the recipients server.

      2. Test to ensure the domain in question has an Mail Exchanger (MX record) configured properly:Open a CMD command prompt on any PC or ServerType nslookup

        Type set q=mx

        Type <remote email domain name>

        If you see the name of a server, then they have an MX record

      3. Use web tools to troubleshoot odd problems:- Check your INBOUND and OUTBOUND SMTP settings using Microsofts fantastic:
        – Use the MXToolBox Supertool
        – Make sure your rDNS (reverse DNS) is setup is correct:

If all this fails, post your question on a forum like and read articles like



  1. Avatar
    Tjday February 18, 2015 at 3:51 pm

    When I got to this step “Type mail from: and enter.”

    I got the following error 500 #5.5.1

    • Ian Matthews
      Ian Matthews March 1, 2015 at 1:07 pm

      Sorry, that is not an error I have run into. I would just Google and see what comes up.

  2. Avatar
    DJM February 12, 2013 at 12:48 pm

    After telnet to port 25 failed, tracert from firewall failed too. Opened ticket with ISP who was able to duplicate issue.

  3. Avatar
    Ron October 29, 2012 at 11:47 am

    In our case, we need to ensure the IP ranges on the Hybrid Receive Connector and the NAT rule for port 25 matched.

    In addition to the original port ranges on the receive connector, we also added the ranges for ‘Microsoft Online Services’ from the microsoft site.

  4. Avatar
    gerd September 19, 2012 at 1:18 am

    if your exchange server don’t have enough space on C: the same Error occurs.

  5. Avatar
    MxToolBox December 28, 2011 at 8:04 am

    Thanks for recommending our SuperTool. I also noticed that you recommended another source for checking you rDNS, our tool does that as well. We’ve worked hard to build an all encompassed tool, if you have any feedback, we’d love to hear it!


    • Ian Matthews
      Ian Matthews December 29, 2011 at 11:08 am

      Your service is excellent and I am very happy to support it.

      Thanks and have a HAPPY NEW YEAR!

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