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Email error 535 5.7.8: Authentication failed (SMTP login)

Your mail program or application could not log in to the SMTP server you send through. This is a problem with your credentials or settings, not with the recipient.

Sender or policy problem

What it means

Your mail program or application could not log in to the SMTP server you send through. This is a problem with your credentials or settings, not with the recipient.

Common causes

  • A wrong username or password.
  • The provider requires an app password or an API key instead of the account password.
  • The wrong port or encryption setting.
  • The account is locked or has two-step sign-in without an app password.

How to fix it

  1. Check the username and password in your settings.
  2. Create an app password or an API key if the provider asks for one.
  3. Use the port and encryption the provider documents, usually 587 with STARTTLS or 465 with TLS.
  4. Sign in to the provider on the web to see whether the account is locked.

How to prevent it

Most bounces come from addresses that were never checked. Verify each address before you send: syntax, domain, mailbox and risk are checked without sending a message. Test a single address with the free email checker or upload a whole list for bulk verification.

Verify an email address free · Clean a whole list · MX lookup · SPF, DKIM and DMARC checker

Frequently asked questions

Does this affect my recipients?

No. The message never left your side.

Why did it stop working suddenly?

Providers rotate security rules, and password changes break saved credentials.

Other error codes

550 5.1.1User unknown (mailbox does not exist) 550 5.7.1Message rejected by policy (blocked) 550 5.7.26Sender not authenticated (SPF/DKIM/DMARC) 550 5.4.1Recipient address rejected: access denied 552 5.2.2Mailbox full (over quota) 553 5.1.3Invalid address syntax

All email error codes