What it looks like in your logs
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient user@example.com not found by SMTP address lookupWhat does 5.1.10 mean?
RFC 7505 registered X.1.10 as "recipient address has null MX". A null MX is a single MX record pointing to a dot, and it is how a domain states that it accepts no email at all. When a sending server sees one, it should not even try to deliver, and it reports 5.1.10 for every address at that domain. Nothing about the code is temporary, so your server will not queue it for another attempt.
In day-to-day sending, you will mostly see 5.1.10 from Microsoft. Exchange Online returns it with RESOLVER.ADR.RecipientNotFound when the address is not in the recipient tenant's directory. For your list, the meaning is the same either way: nothing you send to that address will arrive. The address is not in their directory, perhaps because it was mistyped, the person left, or the mailbox was never created. Exchange looks the recipient up at the edge and rejects it before the message reaches any mailbox.
The distinction matters for cleanup. A Microsoft 5.1.10 is about one mailbox. A null MX 5.1.10 covers the whole domain, which often means a typo turned a real domain into a website-only lookalike, so you can suppress every address at that domain at once.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 5.1.10 | ✓ this code |
Common causes
- The recipient address does not exist in the Microsoft 365 tenant (RecipientNotFound).
- The mailbox was removed or the user left the organization.
- The recipient domain publishes a null MX and accepts no mail by design.
- A typo produced a domain that exists for a website but not for email.
- A parked or brand-protection domain was entered as an email address.
How to fix 5.1.10
- Suppress the address; 5.1.10 is permanent and retries only add to your bounce rate.
- Check the domain with the MX lookup: a single "0 ." record confirms a null MX and every address there can go.
- Look for a near-miss typo against a well-known domain and correct it if you can confirm with the contact.
- Run new imports through an email verifier (ZeroBounce, NeverBounce or SendPulse's email verifier) to catch dead mailboxes and no-mail domains before they bounce.
- Validate at signup that the domain has a usable MX; our guide to DNS records for email explains what a valid record looks like.
5.1.10 questions
What does 550 5.1.10 RecipientNotFound mean?
It is how Microsoft 365 reports that the recipient address does not exist in the destination organization. The directory lookup failed, so the message was rejected. Treat it like a user-unknown bounce: suppress the address and do not retry. It counts toward your hard bounce rate.
What is a null MX record?
It is an MX record with priority 0 and a target of a single dot. RFC 7505 defines it as the formal way for a domain to say it never receives email. It is a deliberate setting, common on website-only and parked domains. You can see it with our MX lookup.
How is 5.1.10 different from 5.1.1?
5.1.1 says one mailbox does not exist. Under the RFC meaning, 5.1.10 says the entire domain accepts no mail. Microsoft blurs the line by using 5.1.10 for single unknown recipients too. Both are hard bounces that call for suppression. Neither will change on retry.
What if my own sending domain has a null MX?
Then receivers may reject your mail, because a domain that cannot receive bounces or replies looks suspicious. That condition is reported as 5.7.27. Replace the null MX with real MX records before sending from that domain. Check with the MX lookup tool first.