What Gmail, Outlook and Yahoo Say When a Mailbox Doesn't Exist
When a mailbox doesn't exist, Gmail replies 550 5.1.1 The email account that you tried to reach does not exist, Microsoft 365 uses 5.1.1, 5.1.10 or 5.4.1 with "Recipient not found" or "Access denied" wording, and Yahoo returns 554 delivery error: dd This user doesn't have a yahoo.com account. The words differ but the meaning is identical: a permanent failure.
Below are the exact strings each provider publishes, side by side, with the codes decoded and what to do about each one. Every quote comes from the provider's own documentation, fetched on 2 October 2026.
How to read a bounce line
Every SMTP rejection has three parts: a reply code, an enhanced status code and free text. Take Gmail's:
550 5.1.1 The email account that you tried to reach does not exist.
| Part | Example | What it tells you |
|---|---|---|
| Reply code | 550 |
First digit: 5 means permanent, 4 means temporary (RFC 5321) |
| Enhanced status code | 5.1.1 |
Class, subject and detail (RFC 3463) |
| Text | "does not exist" | Human-readable, provider-specific, not standardised |
The enhanced code is the part to automate on. RFC 3463 defines X.1.1 as "Bad destination mailbox address", and says, "The mailbox specified in the address does not exist." The free text changes between providers and over time; the enhanced code is meant not to.
Side by side: "this mailbox does not exist"
All three providers return a 5xx permanent failure for an address that does not exist.
| Provider | Reply | Enhanced | Published text |
|---|---|---|---|
| Gmail / Google Workspace | 550 | 5.1.1 | "The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces." |
| Microsoft 365 (Exchange Online) | 550 | 5.1.1 | "Bad destination mailbox address" |
| Microsoft 365 (Exchange Online) | 550 | 5.1.10 | "Recipient not found. SMTP address lookup didn't find the recipient's |
| Microsoft 365 (Exchange Online) | 5.4.1 | "Recipient address rejected: Access denied", described as "The recipient's address doesn't exist." | |
| Yahoo | 554 | "554 delivery error: dd This user doesn't have a yahoo.com account" |
Sources: Google's SMTP error reference; Microsoft's NDRs in Exchange Online and Fix NDR 550 5.1.1 through 5.1.20; Yahoo's SMTP Error Codes page.
Gmail
Google publishes the most complete reference of the three. Its "does not exist" line, 550 5.1.1, is the one most senders have seen. It sits next to several near-relatives that mean something different:
| Reply | Enhanced | Google's text | Meaning |
|---|---|---|---|
| 550 | 5.1.1 | "The email account that you tried to reach does not exist..." | No such mailbox |
| 553 | 5.1.2 | "We weren't able to find the recipient domain..." | No such domain |
| 550 | 5.2.1 | "The email account that you tried to reach is inactive." | Mailbox exists but is disabled |
| 552 | 5.2.2 | "The recipient's inbox is out of storage space and inactive." | Full and inactive |
| 452 | 4.2.2 | "The recipient's inbox is out of storage space." | Full, temporary |
| 450 | 4.2.1 | "The user you are trying to contact is receiving email too quickly. Please resend your message at a later time." | Rate limited, temporary |
The 5.2.1 inactive response matters for list hygiene. The address once worked and has been switched off. It will not come back, and an old list full of these is a sign that the data is stale.
Microsoft 365
Microsoft has three different codes for a missing recipient, depending on where in its system the lookup fails. Its documentation lists:
- 5.1.1, "Bad destination mailbox address". Microsoft gives the likely causes as a mistyped address, an address that "doesn't exist in the destination email system", or a mailbox that moved while the sender's cached entry did not update.
- 5.1.10, "Recipient not found". "SMTP address lookup didn't find the recipient's
." - 5.4.1, "Recipient address rejected: Access denied". Microsoft describes it as meaning "The recipient's address doesn't exist" and links it to Directory Based Edge Blocking, a feature that rejects unknown recipients at the edge, before the message is accepted.
The third one is the one that confuses people, because "Access denied" sounds like a block on the sender. In this context, Microsoft's own description is that the recipient does not exist.
A caveat on scope: these pages document Exchange Online, meaning Microsoft 365 business tenants. Microsoft does not publish an equivalent list for consumer Outlook.com and Hotmail mailboxes on the pages we read, so we have not quoted consumer wording here.
Yahoo
Yahoo documents categories rather than a code-by-code table. Its SMTP Error Codes page says 4xx errors (it names 421 and 451) are temporary and 5xx errors (553 and 554) are permanent, and has a "Recipient does not exist" section that reads:
The Yahoo account that you're trying to send to does not exist... You should not retry delivery of the message. It will never complete successfully. You should remove the email address from your mailing list.
The exact line comes from the sample failure notice on the same page:
554 delivery error: dd This user doesn't have a yahoo.com account (testing123@yahoo.com) [-5] - mta123.mail.mud.yahoo.com
Two things are different from Gmail. The reply code is 554 rather than 550, and there is no enhanced status code in the sample. A bounce parser that only looks for 550 or 5.1.1 will miss Yahoo's version. Match on the 5xx class plus the text, or on your sending platform's own classification.
What to do with each type
Remove permanent "does not exist" failures immediately. Retry temporary ones a limited number of times.
| Bounce | Type | Action |
|---|---|---|
| Gmail 550 5.1.1, Microsoft 5.1.1 / 5.1.10 / 5.4.1, Yahoo 554 "doesn't have an account" | Hard bounce | Suppress now; never retry |
| Gmail 553 5.1.2 (no such domain) | Hard bounce | Suppress; check for a typo in the domain |
| Gmail 550 5.2.1 (inactive) | Hard bounce | Suppress |
| Gmail 552 5.2.2 (full and inactive) | Hard bounce | Suppress |
| Gmail 452 4.2.2 (full) | Soft bounce | Retry over a few days; suppress if it persists |
| Gmail 450 4.2.1 (receiving too quickly) | Soft bounce | Slow down and retry later |
RFC 3463 describes "mailbox full" as something that "should be used as a persistent transient failure", which is why Google sends it as a 4xx.
Our SMTP code pages go into each code in more depth, including 452 4.2.2 and 550 5.2.1.
Catch these before you send
A verifier asks the same question at the RCPT stage without sending anything. The mail server's answer to RCPT TO carries the same codes shown above, so a 550 5.1.1 there means the same thing as a 550 5.1.1 bounce, without the damage to your bounce rate.
That is how the free email verifier classifies addresses: a definite "does not exist" at RCPT is invalid; an accepted address is valid; anything inconclusive, such as a temporary error or a server that accepts every address, is reported as risky. The background is in how to check if an email is valid without sending.
One limit: providers do not always give a definite answer at RCPT time. Some accept the address and bounce it later, and others accept everything. Verification reduces hard bounces sharply but cannot remove them entirely, which is why your sending platform's bounce handling still matters.
Limitations
- Documentation, not live captures. For this post we did not send test mail or open SMTP connections. Every string above is quoted from the provider's published documentation as fetched on 2 October 2026. Real bounces can carry extra text such as server names and session IDs, and wording can change without notice.
- Yahoo's sample is old. The Yahoo example on its error page includes a 2010 date in the message headers. It is the example Yahoo currently publishes, but live wording may differ.
- Microsoft consumer mail not covered. The Microsoft codes are documented for Exchange Online. Outlook.com consumer responses are not quoted here.
The short version
- Gmail:
550 5.1.1 The email account that you tried to reach does not exist. - Microsoft 365:
5.1.1"Bad destination mailbox address",5.1.10"Recipient not found", or5.4.1"Recipient address rejected: Access denied". - Yahoo:
554 delivery error: dd This user doesn't have a yahoo.com account. - All are permanent. Suppress immediately, and parse on the 5xx class and enhanced code rather than exact text.
Common questions
What does Gmail say when an email address doesn't exist?
Google's published SMTP error reference gives 550 5.1.1 with the text: 'The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces.' It is a permanent failure, so the address should be removed from your list.
What does Outlook or Microsoft 365 say for a non-existent mailbox?
Microsoft's Exchange Online documentation lists several codes for this: 5.1.1 'Bad destination mailbox address', 5.1.10 'Recipient not found', and 5.4.1 'Recipient address rejected: Access denied', which it describes as meaning the recipient's address doesn't exist. All three are permanent failures.
Why does Yahoo use 554 instead of 550 for a bad address?
Yahoo's sender documentation treats 553 and 554 as its permanent error codes, and its sample failure notice shows '554 delivery error: dd This user doesn't have a yahoo.com account'. The reply code differs from Gmail's 550, but the meaning is the same: the mailbox does not exist and you should not retry.
Is 'mailbox full' a hard bounce?
Not necessarily. Google returns 452 4.2.2 for a full inbox, which is temporary, and 552 5.2.2 for a full inbox that is also inactive, which is permanent. RFC 3463 describes 'mailbox full' as a persistent transient failure. Retry the 4xx version a few times; suppress the address if it keeps failing or comes back as 5xx.
Verify unlimited addresses for $29.99/month
Real SMTP mailbox checks. No credits, no per-email fees.
Get Started