RFC 2821 and the "implicit MX" rule: Can you really get mail without an MX record?
Posted by Bharat Suneja at 12:53 AM
No MX records = no emailYes, mail can indeed be delivered to a domain even if it does not have MX records.
Let's take a look at what RFC 2821 says:
- Once the domain is identified SMTP must perform a DNS lookup and first attempt to locate an MX record.
- If no MX records are found but an A record is found, it is treated as if it was associated with an implicit MX record with a preference of 0 pointing to that host - also known as the "implicit MX" rule.
- Further, if MX records are indeed found but mail delivery to these fails, the "implicit MX" rule to use the A record should not be used and the situation reported as an error, unless one of the the MX records actually points to the host in the A Record.
- Finally, if more than one MX records are found with same preference values, an SMTP sender should rotate between these.
For instance, we try sending a message to firstname.lastname@example.org, and the domain does not have any MX records, as the following test shows:
Nslookup cannot resolve MX record:
C:\>nslookup -type=mx somedomain.com
primary name server = ns1.ispserver.net
responsible mail addr = dns.ispserver.net
serial = 2006040700
refresh = 28800 (8 hours)
retry = 7200 (2 hours)
expire = 604800 (7 days)
default TTL = 86400 (1 day)
To create an A record for your domain using Windows Server's DNS management console, leave the (host) Name field blank when creating the new A record, and enter an IP address to map it to.
Nevertheless, not having MX records is not a good practice. Think about the implication it has on your DNS server— the sending host will first try to lookup MX records, and then lookup an A record for your domain. You're inviting extra load to your DNS server(s) by not having MX records.
MX records also allow you to route inbound mail to more than 1 hosts using the preference value to load-balance or failover.