Email bounce codes explained, in plain language

You send an email and, a minute later, get a reply you didn't expect: "Delivery Status Notification (Failure)" or "Undeliverable", full of technical text and a code like 550 5.1.1. It looks like gibberish, but it's actually a precise message from the receiving mail server explaining exactly why your email didn't arrive.

Once you know how to read it, most bounces take seconds to diagnose.

How to read a bounce message

Bounce messages vary between providers, but they all contain the same key piece of information, usually labelled something like "Diagnostic code", "Remote server returned" or "The response from the remote server was". It looks like this:

550 5.1.1 <omar@example.com>: Recipient address rejected: User unknown

That line has three parts:

• 550: the basic reply code. These come from SMTP, the language mail servers use to talk to each other (explained in How email actually works).
• 5.1.1: an "enhanced" status code that gives more detail.
• The text: a human-readable explanation. Each server words this differently, and it's often the most useful part.

The first digit tells you almost everything

If you receive a warning like "Delivery delayed" or "still trying", that's a 4xx at work. Only when the sender finally gives up, often after 24 hours to 5 days, do you get the final failure notice.

Common codes and what to do

550 5.1.1 — "User unknown" / "Recipient not found"

What it means: the address doesn't exist on that server. Microsoft's systems often show this as 5.1.10.

What to do: check the address for typos, especially in the part before the @. The person may have left the company, or the address may have been deleted. Confirm the correct address another way.

550 5.1.2 — "Host unknown" / "Domain not found"

What it means: the domain after the @ doesn't exist or doesn't accept email.

What to do: look for a typo in the domain: gmial.com, outlok.com, .con instead of .com. If it's a company domain, it may have expired or changed.

452 / 552 with 4.2.2 or 5.2.2 — "Mailbox full"

What it means: the recipient has run out of storage. It's their problem, not yours.

What to do: contact them another way and let them know. If it happens to you, see how to free up Gmail storage.

552 5.3.4 or 5.2.3 — "Message too large"

What it means: your email, usually because of attachments, is over the receiving server's size limit. Many limits are around 20 to 25 MB, and attachments grow by about a third in size when they're encoded for email.

What to do: share large files as a link (Google Drive, OneDrive, Dropbox) instead of attaching them.

421 / 450 / 451 — "Try again later"

What it means: a temporary problem: the server is busy, is limiting how fast it accepts mail from your server, or is using greylisting, which deliberately delays the first message from an unfamiliar sender.

What to do: usually nothing. Your provider will retry automatically, and the email typically gets through within minutes or hours.

550 5.7.1 / 554 5.7.1 — "Message rejected" / "Access denied"

What it means: a policy rejection. The receiving server decided not to accept your message. Common reasons: it looks like spam, your server or domain is on a blocklist, the recipient's organisation only accepts mail from certain senders, or the recipient blocked you.

What to do: read the text after the code, which usually says more. Try sending a short plain message with no links or attachments. If you run your own domain, check whether your sending IP is on a public blocklist.

550 5.7.26 — "Unauthenticated email is not accepted"

What it means: this is Gmail telling you the email failed authentication checks, typically because the sending domain has a DMARC policy and the message didn't pass SPF or DKIM in the right way.

What to do: if you own the domain, your SPF, DKIM or DMARC setup needs fixing. A common cause is a new sending service (newsletter tool, website form) that isn't included in your records. Our guide SPF, DKIM and DMARC, explained simply walks through a safe setup.

550 5.7.25 — "PTR record" / reverse DNS missing

What it means: the server that sent your email doesn't have a proper reverse DNS record, which many large providers require.

What to do: this is a server configuration issue. If you use a normal email provider, contact them. If you run your own mail server, set up reverse DNS with your hosting provider.

550 5.4.1 — "Recipient address rejected: Access denied"

What it means: common with Microsoft 365. The recipient's organisation checked its directory and rejected the address, often because it doesn't exist there, or because the organisation restricts who can email it.

What to do: confirm the address with the recipient. If it's correct, their IT team may need to allow your address.

553 — "Mailbox name not allowed"

What it means: the address format was rejected, or your server isn't allowed to send for the address you used as the sender.

What to do: check the recipient address for invalid characters. If you're sending through an app, check that the sending address matches your account.

Bounces for emails you never sent

If your inbox is filling with bounce messages for emails you didn't write, a spammer is probably using your address as the fake sender. This is called backscatter. Check your Sent folder: if the emails aren't there, your account wasn't used; your address was forged. Our guide to sudden spam increases explains what to do, and signs your email has been hacked helps if the emails are in your Sent folder.

No bounce, but no delivery either

Sometimes an email simply disappears: no bounce, no reply. Usually it was delivered, but to the recipient's spam folder, or it was accepted and then silently filtered. Ask the recipient to check their spam and junk folders, and search for your address. If it keeps happening with many recipients, your domain's reputation or authentication setup may need attention.

Quick reference

Bounce messages feel like the internet shouting at you in code. Read the first digit to know whether to wait, then the text to know what to fix, and you can solve most of them yourself.