550 5.1.10: Recipient not found
550 5.1.10 is mostly seen from Microsoft-hosted mailboxes and means the recipient address was not found. In practice it is the Microsoft version of "this mailbox does not exist". Treat the address as dead.
- Bounce type
- Hard bounce (permanent)
- Usually caused on
- The recipient's side
- Terlido report label
- Unrecognised
What the standard says
RFC 5321 defines 550 as "mailbox unavailable". For the enhanced code, RFC 7505 defines X.1.10 as "Recipient address has null MX", which means the recipient's domain publishes a record saying it accepts no mail. Microsoft uses 5.1.10 with a different meaning: its address lookup did not find the recipient. Check the text after the code to know which one you have.
What the mailbox providers say
- Microsoft 365 / Outlook (5.1.10): “Recipient not found” Source
Common causes
- The mailbox on a Microsoft 365 domain does not exist, because of a typo or because the person left.
- The recipient's tenant has the domain set up but never created that user or alias.
- An alias or shared mailbox that was removed by the administrator.
- Under the RFC 7505 meaning: the recipient's domain publishes a null MX and never accepts mail at all.
- A recently renamed user in Microsoft 365, where the old address no longer works unless the administrator kept it as an alias.
- An address that came from an old email signature or an old press page for a company that has since moved to Microsoft 365.
How to fix it
- Read the full bounce text. "Recipient not found" points to a missing mailbox, and a null MX note points to a domain that takes no mail.
- Check the address for typos against a reliable source and fix it if you can confirm the right one.
- Remove the address from the list if you cannot confirm it.
- Look up the domain's MX records to see whether it accepts mail at all.
- If the person changed their name or role, look for their current address on the company site or profile before you drop them.
- Separate the two meanings in your notes. A missing recipient is a list problem, while a null MX is a domain that never takes mail.
Is it a deliverability problem?
Treat this like 5.1.1. A few are normal on a cold list, and a high share means the list needs cleaning. Your report may show it as "Unrecognised" rather than "Invalid recipient", because 1.10 is not in the short list of addressing codes that the report groups together. That is only a label. The bounce is still permanent. The code is one of those where the number alone does not settle the meaning, which is why the text matters. Whichever meaning applies, the address is dead for your purposes, and removing it keeps your hard bounce rate down.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 5 is a hard bounce. In the campaign Reports tab this one is grouped as "Unrecognised".
On a hard bounce Terlido adds the address to the workspace blocklist with the reason "hard bounce", so no campaign emails it again. The lead stops in that campaign as bounced, its lead score goes down by 15, and the bounce counts toward the campaign's bounce rate.
Once a campaign has at least 20 sends, it pauses itself when hard bounces divided by sends goes over the campaign's maximum bounce rate. The default is 5%. If the cause was on your side and you fixed it, you can remove the entry on the Blocklist page of your account settings and try again.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email list format checkerReview CSV structure, empty addresses, and basic email syntax without contacting mail servers.
- MX lookupShow a domain's mail exchanger records.
Related error codes
- 550 5.1.1: Recipient address does not exist
- 550 5.1.2: Recipient domain does not exist
- 550 5.2.1: Mailbox disabled or not accepting mail