email and reverse DNS lookups

B

big daddy fatsacks

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we are implementing a customer management system thyat is web-based and hosted at another organization. this thing will be using mailman or something similar to send emails. i was told to add a new MX record to our DNS zone for the mailman server which is in a different domain. so here's the scenario:
mailman.otherdomain.com

new DNS record for mydomain.org as
MX 100 mailman.mydomain.org
mailman.mydomain.org CNAME mailman.otherdomain.com

the idea was that when mail servers do a reverse DNS lookup to determine if they are getting mail from a valid mydomain.org mail server that they would get the response mailman.mydomain.org and all would be good. but instead the CNAME is being returned mailman.otherdomain.com. is this going to be a problem? will spam filters reject this mail since it is From [email protected], but being delivered by mailman.otherdomain.com?

my understanding is that it does not matter as long as the name of the server sending the email is listed as a valid mail exchanger for mydomain.org. am i wrong? the workaround would be to replace the CNAME record with an A record that points to the actual address of mailman.otherdomain.com.

your thoughts?

TIA,
billy ocean
 
Wow, I just got through looking into this. If I go here and do a lookup on my domain name, there are these three entries in the report:


  1. OK. Looking up your MX record did not just return a CNAME. If an MX record query returns a CNAME, extra processing is required, and some mail servers may not be able to handle it.
  2. OK. There appear to be no CNAMEs returned for A records lookups from your MX records (CNAMEs are prohibited in MX records, according to RFC974, RFC1034 3.6.2, RFC1912 2.4, and RFC2181 10.3).
  3. OK. All of your MX records are host names (as opposed to IP addresses, which are not allowed in MX records).
So going by #2, you need an A record to go with an MX record...CNAME's are out. You might want to look into an MXE (mail exchange - easy) record too...that's a mapping to an IP address. Hope this helped!
 
hey thanks. that's great information. i'll prolly just use an A record and hope the server i'm pointing to doesn't change IPs a lot (i seriously doubt it will). thanks a lot.
 
hahahahaha. what a day. i called our web host who requested these specific changes be made to our DNS. i asked to speak with their network guy who said he was consulted about what changes they were supposed to ask us to make . . . . but that something may have gotten lost in translation between he and the project leader. boy howdy.

they were supposed to request 1 entry be made for:
mailman.mydomain.org IN MX 20 mailman.otherdomain.com

no A record. no CNAME record. this MX wasn't even supposed to show up for mydomain.org. only if you look up mailman.mydomain.org.

anyway, i'm all straight now. thanks a bunch.
 
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