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Somebody has 2 e-mail addresses: bob@domainname.com.br and bob@domainname.com . The DNS records for these domains look like this:

domainname.com.br

$ dig any domainname.com.br
;; ANSWER SECTION:
domainname.com.br.  86179   IN  SOA ns1.domainname.com.br. suporte.domainname.com.br. 2010081200 20000 4000 1409600 86400
domainname.com.br.  86179   IN  MX  10 domainname.com.br.
domainname.com.br.  86179   IN  A   177.39.XXX.YY
domainname.com.br.  85342   IN  NS  ns1.domainname.com.br.

;; AUTHORITY SECTION:
domainname.com.br.  85342   IN  NS  ns1.domainname.com.br.

;; ADDITIONAL SECTION:
domainname.com.br.  86179   IN  A   177.39.XXX.YY
ns1.domainname.com.br.  85342   IN  A   177.39.XXX.YY

domainname.com

$ dig any domainname.com
;; ANSWER SECTION:
domainname.com. 37036   IN  SOA ns1.domainname.com.br. mattana.domainname.com.br. 1229685618 10800 3600 604800 38400
domainname.com. 37036   IN  A   177.39.XXX.YY
domainname.com. 37036   IN  NS  ns1.domainname.com.br.

;; AUTHORITY SECTION:
domainname.com. 37036   IN  NS  ns1.domainname.com.br.

;; ADDITIONAL SECTION:
ns1.domainname.com.br.  85036   IN  A   177.39.XXX.YY

The domainname.com doesn't have a MX record. However, when I send an e-mail to bob@domainname.com, it gets delivered, and the receiver reply using the bob@domainname.com.br address. The e-mail sent to bob@domainname.com is probably being redirected to bob@domainname.com.br. How is this possible, given that domainname.com doesn't have a MX record?

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1 Answer 1

up vote 36 down vote accepted

According to the Section 5 of RFC 5321, if no MX record is present mail servers should fall back to the A record for the domain. This is probably what's happening.

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9  
And since the A record for both domains appear to resolve to the same IP address (despite the obfuscation) my guess is that there's an SMTP server at that ip address that accepts emails for both domains and deilvers them to a mailbox for bob@domainname.com.br. –  joeqwerty Jan 19 '13 at 17:27

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