Here's a strange one. You send to an address, the domain still shows a webpage, and the message bounces anyway. Because a domain can be alive on the web and dead for mail. Let's talk about dead domains, and how to spot them before they wreck your reputation.

A dead domain is a domain that no longer runs a working mail server. It might still be registered. It might even load a parked page. But send mail to anything@deepcurrent.io and there's nothing on the other side accepting the connection on port 25. The message has nowhere to land.

There are a few flavors of dead, and they fail slightly differently. No MX record and no A record to fall back on: the sending server gets a permanent lookup failure. Or the MX points at a hostname that no longer resolves. Or the MX resolves but nothing answers on port 25, the server got shut off, the company folded. Or the domain expired entirely, and once DNS stops caching, everything goes dark.

All of these produce a hard bounce, the mail server's way of saying "do not try again, ever." You'll see a 550 code, often 550 5.1.2 or "Host unknown." The SMTP standard, RFC 5321, treats any 5xx reply as permanent. That's the definition of a hard bounce.

If this is the first time bounce codes have actually made sense, subscribe, we're doing the whole deliverability playbook one question at a time.

Here's the sneaky part: dead domains accumulate quietly. The address was real when someone typed it in. Then the company folded, the side project got abandoned, or the owner stopped paying the registrar. Nothing notifies you. You just keep mailing it for months until you look at your bounce logs. B2B lists rot fastest, because small businesses fold all the time. Bought or scraped lists start out worse.

The good news: catching them is cheap and fast, and it happens before you ever try the mailbox. Check the DNS first. Query the domain for MX, then A. If both are missing, it's dead at the DNS layer, free to check, no send needed. Only then does a full validator bother opening an SMTP connection to see if the server actually answers. Cheapest check first, most expensive last.

Why bother? Because every message to a dead domain is a hard bounce, and hard bounces stack up. A handful in a 50k list won't move the needle. But three thousand of them will get you throttled at the big mailbox providers, and that hurts delivery for every clean address on your list, not just the dead ones.

So: dead domain means no working mail server, guaranteed hard bounce, and it's cheap to catch with a DNS check before you send. When you find one, remove it. Don't retry it. Don't park it in a "maybe next quarter" segment. They're not coming back. If your list is old and you'd rather not do this by hand, cleaning it is exactly what RME Clean does (hi).

Next up: the exact record a dead domain is missing, the MX record, and what happens when it's gone.

What is a dead domain, and why does mail to it always bounce? A dead domain no longer runs a working mail server, even if it still loads a parked webpage. Send to it and you get a hard bounce, a permanent 550 reply (often 550 5.1.2 or "Host unknown"), because there's nothing accepting connections on port 25. In this video: the flavors of dead (no MX, dead MX host, nothing on port 25, expired domain), how they sneak onto your list quietly, the cheap DNS check that catches them before you send, and why 3,000 dead domains in a 50k list can get you throttled.

0:00 The domain loads, the mail bounces

0:20 What a dead domain actually is

0:45 The flavors of dead

1:15 The real bounce code (550 5.1.2)

1:50 How they sneak onto your list

2:20 The cheap DNS check that catches them

2:50 Why you can't ignore them

Next: What is a domain with missing MX records? → [link 005.008.005]

Full written guide → reviewmyemails.com/emailalmanac/005-list-management-and-hygiene/domain-and-dns-level-checks

#email #deliverability #listhygiene