Configure sendmail to not check for PTR

Posted on 2005-05-03
Last Modified: 2013-12-17
Can someone point in the right direction to configure Sendmail 9.3, running on Slackware 10.1, to NOT check for the existance of a PTR record on the senders ip address? I realize this goes against the grain but one of our customers is having problems getting reverse DNS on his connections and has a wide range of dynamic ip's to simpy try to allow by ip address.

Thanks in advance for any assistance you can offer.


Question by:GCaron
    LVL 40

    Expert Comment

    I'm not sure I understand what you would be trying to accomplish by that. Sendmail will always attempt a reverse lookup on an SMTP connection, but it doesn't matter if that works or not. If it doesn't the Received headers will just reflect the IP rather than the hostname.

    Author Comment

    The current configuration of sendmail rejects emails being sent by senders that do not have any reverse dns on the ip address that they are sending from. This is what I am attempting to remove from the configuration.

    LVL 34

    Accepted Solution

    See -->

    Specifically, you want -->  FEATURE(`accept_unresolvable_domains')

    If this is happening for just a few hosts, it would be better to use an access map with the "Connect:" keyword as described a bit further down on that web page.
    LVL 40

    Expert Comment

    Are you talking about mail to an account on the server being rejected, probably with a "domain of the sender doesn't resolve"? That would be an indication that the envelope sender addres doesn't contain a valid domain and "FEATURE(`accept_unresolvable_domains')" would fix that and the risk of a bit more spam. A reverse IP lookup is not used for this test. Sendmail queries DNS with the sender's domain to see if said domain exists.

    I suspect that you are talking about mail being relayed through the server. In that case the best solutions are to:

    1) List the IP's in the access map with RELAY if, and only if, those IP's are exclusively used by your customer.

    2) Enable SMTP AUTH and have your customer set all of their mail clients up to authenticate the SMTP connection.

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