A null MX cannot take the bounce from a solo ad

Answer A null MX means the domain has announced that it does not accept mail. RFC 7505 is the specification for that record. Replies and bounces aimed at a domain with a null MX are supposed to die. Before you pay for a solo ad, ask whether the bounce domain and the from-domain can actually receive a letter.

The note that started this: RFC 7505, A Null MX No Service Resource Record, RFC 7505.

Yesterday's scorecard was about someone else's revenue. Today is about a quieter failure. A seller can show you a from-name, a screenshot, and a price, and still point the return path at a domain that has told the rest of the internet to stop knocking. You will not see that in the pitch deck. You will see it in DNS, if you look.

RFC 7505 is not a July product note. It is the document that standardized a simple announcement: this name does not accept mail. Operators publish it so other servers stop trying, and stop queueing, and stop guessing. That is a kindness when the domain is a website that was never meant to have a mailbox. It is a problem when the domain is the one your bounces are supposed to reach. A solo ad that fails into a void looks, from the buyer's side, like a send with no bad news. No bad news is not the same as a clean file.

What is a null MX on a sending domain?

It is a DNS record that announces the domain does not accept mail. RFC 7505 describes that record. Mail addressed to the domain is not supposed to be delivered.

DNS is the phone book. An MX record is the line that says which machine accepts mail for a name. A null MX is the line that says there is no such machine, on purpose. Sending software that honors the record should stop. It should not hunt for an A record and hope. The specification exists so that hope is not the delivery policy.

People mix this up with a domain that has no MX because nobody filled in the form. An empty record and a null record are different statements. Empty can mean unfinished. Null means finished, and the finish is refusal. On a solo ad, either one should stop the order until a human explains where failure goes. The null record is just the clearer of the two refusals.

• The from-domain is written on the quote.
• The bounce domain is a second line, even if it matches the first.
• Someone has looked up whether that domain accepts mail.
• A null MX on the bounce domain stops the order.

Why does a null MX matter on a solo ad?

Bounces and replies need a domain that can receive them. If the address that should hear about failure publishes a null MX, those messages die. You cannot audit a send you cannot hear back from.

A solo ad is one dedicated email to a list you do not own. The reader who hates it may reply. The mailbox that does not exist may bounce. Both of those messages have to land somewhere if you want a count that means anything. A seller who points them at a domain with a null MX has built a send that cannot report its own dead addresses. The delivered number then includes people who were never going to see it, because nobody received the notice that they were gone.

This is also how a buyer gets a false comfort. The complaint mailbox never fills, so the list looks quiet. Quiet can mean the readers were fine. Quiet can also mean the readers had nowhere to send the complaint that you would have counted. Safe solo ads sellers are the ones who can name the mailbox and show that it is a real box, not a record that tells the world to go away.

What should you ask before you pay for the send?

Ask which domain receives bounces, ask whether that domain publishes a mail server record rather than a null MX, and ask who reads that mailbox.

You do not need the seller to recite the RFC number. You need a noun. A domain. A record that points at a server. A person who opens the mailbox when a hard failure arrives. If the domain is only a landing page, put the bounce on a domain that accepts mail, and write that domain into the order.

Check the from-domain the same way if replies are supposed to come back to it. A pretty from-name on a domain that publishes a null MX is a costume. The reader hits reply and the letter dies. The buyer hears that the list is engaged because nobody wrote back. Ask the question on the second day of the month, while the quote is still a document. A domain that cannot take the bounce cannot tell you the truth about the send.