SMTP delivery or non-delivery with multiple MX records

Posted on 2010-01-05
Last Modified: 2012-05-08
This question is concerning the typical default delivery behavior for email servers. The default delivery behaviors for Exchange would be a sufficient answer to the question.

An organization has 2 MX records for its email domain.

MX priority 5 is routed to an Exchange server on IP    (not actual address)
MX priority 10 is routed to a fail-over internet connection at this organization but no SMTP server ever answers on this IP.

If the server at MX priority 5 is off-line temporarily and MX priority 10 is always dead, will the typical  sending system go into a retry schedule or will it report a non-dilivery without retries.  

I am trying to determine if the MX configuration above creates a delivery reliability issue or not by having a 2nd priority MX record that is always dead.
Also, is there ever a good reason to have a 2nd MX record that goes to a dead-end.  I know that some admin will have a dead 1st priority MX record to drop some spam senders that don't bother trying the 2nd or 3rd MX addresses.    
Question by:swb_mct
    LVL 57

    Accepted Solution

    If the 2nd MX record points to dead end, then there is no valid reason to have it.  It really does nothing but waist time/resources on the sender's side when the host for the 1st MX record is down.

    Most SMTP servers will not fail a attempted e-mail send because of one failure.  Typically they are configured to retry every so often and after "time period #1" will send the sender a "mail delivery is delayed" and after "time preiod #2" will send a "can't deliver this mail."

    It used to be that time period #1 was 24 hours and time period #2 was 5 days by default.  I have no clue what defaults are today.
    LVL 76

    Expert Comment

    by:Alan Hardisty
    Having a secondary MX record (when it is working) can open the system up to more abuse from spammers as they know that the secondary MX is likely to be less secure and will actively target the secondary server.
    If your 2nd MX record is not working, get rid of it.  If mail cannot be received by your primary server, some mail will bounce and some will be retried, but that is out of your control.
    Most servers will retry mail for up to 3 days, but this varies and some don't even bother retrying, although this is the exception rather than the rule.
    Spammers will target any MX record you publish - they don't follow the normal rules of MX priority, so using dummy servers on primary MX records, or secondary MX records is a waste of time.
    Setup your primary mail server reliably and you won't have any problems.  Install as much built-in redundancy as you can (dual CPu's, dual NIC's, dual Power Supplies, Dual RAID Controllers, RAID 5 minimum drives, Multiple Exchange servers etc) and your environment will stay up.

    Author Comment

    OK, the main point of my question may not have not been too clear, because my main reason for asking got paritally lost in the answers.

    To focus my question better . . . if an organization has a 2nd prioriy MX record that never answers . . because there is no smtp server at that address, will the typcial settings on Exchange or other commercial email systems be more likely to give up on delivery without a retry sequence if the 1st prioriity server is temporarily off-line.

    My guess is "No", but I don't know for sure if that is correct.  thanks  

    LVL 76

    Assisted Solution

    by:Alan Hardisty
    Your guess is correct.  It will try the primary and if no response will try the secondary, then if no response it will retry.

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