Email error code lookup

Got an error from your email app, printer, scanner or script? Paste it below to find out what it means and how to fix it.

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We don’t recognize that error yet. Try searching for just the code (such as 5.7.139) below, or check the troubleshooting guide.

All error codes

Any

530 5.7.0 Must issue a STARTTLS command first

SMTP · The server requires an encrypted connection, but your app tried to continue without enabling encryption.

WRONG_VERSION_NUMBER SSL wrong version number

SMTP, IMAP, POP · Your app tried to start an encrypted (SSL/TLS) connection, but the server was expecting a plain connection that is upgraded later with...

554 5.7.1 / 550 5.7.1 Relay access denied / Unable to relay

SMTP · The server refused to pass your message on to another domain, because your app didn’t sign in (or isn’t allowed to relay).

554 5.7.1 / 550 5.7.1 Blocked using a blocklist (DNSBL)

SMTP · The receiving server rejected the message because the sending IP address or domain is on a spam blocklist.

550 5.1.1 User unknown / mailbox does not exist

SMTP · The recipient’s server says the email address doesn’t exist.

552 5.2.2 / 452 4.2.2 Mailbox full / over quota

SMTP · The recipient’s mailbox has run out of storage space.

552 5.3.4 Message too large

SMTP · The message, including attachments, is larger than the server allows.

450 / 451 4.7.1 Greylisted – try again later

SMTP · The receiving server temporarily refused the message to check that the sender retries, as real mail servers do. It is not a permanent failure.

535 5.7.8 Authentication failed Sign-in

SMTP · The SMTP server rejected the username and password your app sent.

Gmail

534-5.7.9 Application-specific password required Sign-in

SMTP, IMAP, POP · The Google account has 2-Step Verification turned on, so its normal password can’t be used in email apps.

535-5.7.8 Username and password not accepted Sign-in

SMTP · Google rejected the username and password your app sent.

534-5.7.14 Please log in with your web browser Sign-in

SMTP, IMAP · Google blocked the sign-in because it looked unusual, and wants you to confirm it’s you in a web browser.

[AUTHENTICATIONFAILED] Invalid credentials (Failure) Sign-in

IMAP · Gmail’s IMAP server rejected the sign-in.

550-5.7.26 Sender is unauthenticated

SMTP · Gmail rejected the message because it passed neither SPF nor DKIM – or failed the sending domain’s DMARC policy.

550-5.7.25 Missing PTR (reverse DNS) record

SMTP · The sending server’s IP address has no reverse DNS (PTR) record, or it doesn’t match the server’s hostname.

421-4.7.28 / 550-5.7.1 Unusual rate of unsolicited mail

SMTP · Gmail is rate-limiting or rejecting email from your IP address or domain because recipients have been marking it as spam.

550 5.4.5 Daily sending limit exceeded

SMTP · The Google account has sent more email in the past 24 hours than Gmail allows.

454-4.7.0 Too many login attempts Sign-in

SMTP · Google is temporarily blocking sign-ins from your app because of repeated attempts.

Microsoft 365

535 5.7.139 Basic authentication is disabled Sign-in

SMTP · Microsoft 365 rejected your app’s username and password because basic authentication (signing in with just a password) is switched off...

535 5.7.139 SMTP AUTH is disabled for the tenant or mailbox Sign-in

SMTP · Authenticated SMTP (SMTP AUTH, also called client submission) is turned off for your whole Microsoft 365 organization, or for the...

535 5.7.139 Blocked by security defaults Sign-in

SMTP · Your Microsoft 365 organization uses security defaults, which block all sign-ins that use basic authentication (a username and password...

535 5.7.139 Sign-in blocked by Conditional Access or MFA Sign-in

SMTP · The password was accepted, but a Microsoft Entra policy – usually Conditional Access or multi-factor authentication – prevented the sign-in.

535 5.7.139 User credentials were incorrect Sign-in

SMTP · Microsoft 365 did not accept the username and password your app sent.

530 5.7.57 Client not authenticated to send mail Sign-in

SMTP · Your app tried to send an email through smtp.office365.com without signing in first.

421 4.7.66 TLS 1.0 and 1.1 are not supported Sign-in

SMTP · Your app is using an outdated version of TLS encryption that Microsoft 365 no longer accepts.

BAD User is authenticated but not connected User does not have access to the mailbox

IMAP, POP · The OAuth sign-in succeeded, but the signed-in identity does not have access to the mailbox you asked for.

-ERR Logon failure POP logon failure Sign-in

POP · Microsoft 365 rejected a POP sign-in. Password sign-in is no longer accepted for POP – only OAuth 2.0 works.

554 5.2.252 / 550 5.7.60 Send As permission denied

SMTP · You signed in as one user but tried to send from a different address, and that user does not have permission to send as it.

550 5.7.520 External forwarding is not allowed

SMTP · A forwarding rule tried to send email outside your organization, but Microsoft 365’s outbound spam policy blocks automatic external forwarding.

550 5.7.708 / 5.7.705 Tenant or IP blocked from sending

SMTP · Microsoft 365 has restricted sending from your organization or IP address, usually because of suspicious or high-volume outbound email.

550 5.4.1 Recipient address rejected: Access denied

SMTP · The recipient’s Microsoft 365 organization doesn’t recognize the address you are sending to (Directory-Based Edge Blocking).

550 5.1.10 Recipient not found

SMTP · The recipient’s email address doesn’t exist in the destination Microsoft 365 organization.

550 5.7.509 Message failed DMARC with a reject policy

SMTP · The message failed DMARC authentication, and the sending domain’s DMARC policy tells receivers to reject such messages.

Microsoft 365 / Exchange

535 5.7.3 Authentication unsuccessful Sign-in

SMTP · The Exchange or Microsoft 365 server rejected your app’s sign-in, without giving a specific reason.

Microsoft 365 / Outlook.com

AUTHENTICATE failed AUTHENTICATE failed / LOGIN failed Sign-in

IMAP, POP · Microsoft’s IMAP or POP server rejected your app’s sign-in. Basic authentication (a password) is permanently disabled for IMAP and POP...

550 5.7.606–649 Banned sending IP

SMTP · Microsoft has added the sending server’s IP address to its own block list.

451 4.7.500 Server busy – try again later

SMTP · Microsoft is temporarily throttling email from the sending IP address. This is a temporary error and the sending server will retry automatically.

Yahoo / AOL

535 5.7.0 (#AUTH005) Too many bad auth attempts Sign-in

SMTP · Yahoo rejected the sign-in after repeated failures. Yahoo and AOL require an app password (or OAuth) for email apps.

Why are so many apps suddenly showing sign-in errors?

Microsoft 365 and Google Workspace have switched off basic authentication – signing in to IMAP, POP or SMTP with just a username and password. Apps must now use OAuth 2.0 instead. Email apps, printers, scanners, line-of-business software and scripts that only support passwords stop working, and show errors such as 535 5.7.139, AUTHENTICATE failed or 534-5.7.9.

Errors marked Sign-in are usually caused by this change. Auth-Email fixes them without changing your app: it signs in to your email provider with OAuth on your app’s behalf, while your app keeps using a normal username and password.

Fix sign-in errors without replacing your app

Auth-Email lets any app, device or script that uses IMAP, POP or SMTP keep working with Microsoft 365, Outlook, Gmail and other OAuth-only email accounts. Start with a free trial.

Read the docs Try Auth-Email →

Frequently asked questions

Why did my email app suddenly stop sending or receiving?
The most common reason is that your email provider has switched off basic authentication (signing in with just a username and password). Microsoft 365 and Google Workspace now require OAuth 2.0 for IMAP, POP, and SMTP. Older apps, printers, scanners and scripts that cannot use OAuth stop working, and usually report an error such as 535 5.7.139 or 534-5.7.9.

What do the numbers in an SMTP error mean?
The first three digits are the reply code: 2xx means success, 4xx is a temporary failure (the sender should try again later) and 5xx is a permanent failure. The second number, such as 5.7.139, is an enhanced status code: its first digit repeats the class, the second gives the category (for example, 7 means security or policy) and the third is the specific reason.

Is the error caused by my app or my email provider?
Authentication errors (codes starting 5.7 or 535) usually mean the provider rejected the way your app signed in. Recipient errors (5.1.x) relate to the address you are sending to. Use our email server checker to see which sign-in methods your provider offers.