What the receiver is telling you

Your envelope-from domain publishes a null MX — a single MX record with priority 0 pointing at '.' — which is the standard way to declare that a domain never receives mail. The receiver, quite reasonably, refuses to accept mail from an address that cannot receive a bounce. This appears when someone sets a null MX on a sending-only domain (a defensive step against spoofing) and then uses that domain as a bounce address, or when a tool's return path was pointed at a domain configured never to receive.

Why a policy block must not be treated as a bad address

Sending tools file every 5xx under "bounced", and the natural reflex is to scrub the address. For a policy block that is exactly wrong. The recipient exists and would have received the mail; the receiver refused it because of your domain, IP, authentication or content. Scrubbing throws away a lead and leaves the cause in place, so the next send produces the same block against the next thousand addresses. The right response is to stop sending from the affected domain, find what the receiver named, fix it, and resume at low volume.

What to do

1. Find the envelope-from domain in the bounce or your tool's settings.
2. Either give that domain a real MX that accepts bounces, or change the tool's return path to a domain that has one.
3. Sending domains in a cold email fleet must be able to receive — replies and bounces both need somewhere to land. Null MX is for domains that send nothing.

don't Do not remove the null MX from a domain that genuinely sends nothing. Fix the return path instead.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions