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MX Records: Are they really needed?

Posted by Milind Naphade on 18/05/2011

Sound a weird question? Yes, it shocked me too. One of my customers run Symantech BrightMail Gateway for their inbound email handling. This appliance has multiple accepted domain names configured. We were amazed to see some domain names not having an associated MX records yet they were receiving emails from internet. I though of digging down more into this just because it was interesting 🙂

For most of us the equation is extremely simple, No MX no email. But I learned something extremely interesting today. RFC 2821 says you can receive emails even if you do not have a MX record for your domain name.

Below text comes from  RFC 2821

5. Address Resolution and Mail Handling Once an SMTP client lexically identifies a domain to which mail will be delivered for processing (as described in sections 3.6 and 3.7), a DNS lookup MUST be performed to resolve the domain name [22]. The names are expected to be fully-qualified domain names (FQDNs): mechanisms for inferring FQDNs from partial names or local aliases are outside of this specification and, due to a history of problems, are generally discouraged. The lookup first attempts to locate an MX record associated with the name. If a CNAME record is found instead, the resulting name is processed as if it were the initial name. If no MX records are found, but an A RR is found, the A RR is treated as if it was associated with an implicit MX RR, with a preference of 0, pointing to that host. If one or more MX RRs are found for a given name, SMTP systems MUST NOT utilize any A RRs associated with that name unless they are located using the MX RRs; the "implicit MX" rule above applies only if there are no MX records present. If MX records are present, but none of them are usable, this situation MUST be reported as an error.

This clearly means if the IP address associated with your exchange server is the same that resolves to  your domain name’s IP address and you do not have any MX records configured in your DNS, you can still receive emails from the sending server compliant to RFC 2821.

Exchange Server 2007 and Exchange Server 2010 both applications are compliant to the mentioned RFC. Although I did not test this scenario yet this information might help you resolving your questions if you faced similar issue like I did.

5 Responses to “MX Records: Are they really needed?”

  1. Peter said

    While this may be true, what is the advantage to not creating MX records? All it does is save you the 10 minutes it takes to create the record the one time you need to do it.
    Given that we know not every sender is going to follow RFC2821 to the letter, it seems like a false savings to me.

  2. I agree to the fact it is a false saving but this is not something that is recommended. This is just an information to the people who might have ever wondered about this situation (Just like me 🙂 )
    If you are asking me about a good part of it then I will say, ” It will be useful for the people with no experience at all. They will still be able to get their messaging servers running.” Yet, I appreciate your comment on this post. Discussions make it easier to decide usability of any feature. Cheers!

  3. Peter said

    “It will be useful for the people with no experience at all. They will still be able to get their messaging servers running.” – On the contrary, I would suggest that someone with no messaging experience learn a little bit first so they can understand what is going on and configure the server properly from the start. If you plug it in and it just works, you never learn what is really going on behind the scenes and you are completely unprepared when something goes wrong. Or in this case, someone has a server that isn’t RFC2821 compliant and their mail keeps bouncing back and you have absolutely no idea why because you know nothing about DNS or MX records.

  4. Whoa… This is definitely a must know…
    I think we should kick our trainers for never “let-know” this stuff… Im sure even they are not aware… lol… Good one Milind…

  5. Thanks Ratish. Sure, I was surprised to read about it too.

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