Is 535 5.7.8 permanent or temporary?

535 5.7.8 is a permanent (5.x.x) failure — the server will not accept the message as-is, and retrying without changing anything won't help. You need to fix the underlying cause.

Is this my problem or the recipient's?

It depends on the cause — wrong username or password is the most common. Work through the causes below to find which applies to you.

How do I check my sending setup?

Run a free EmailHealth check on your domain — it verifies SPF, DKIM, DMARC, blacklist status, and more in a couple of seconds, which covers the most common causes of delivery failures.

535 5.7.8 — Authentication failed (bad SMTP credentials)

535 5.7.8 is a permanent SMTP reply meaning authentication failed (bad smtp credentials). 5.7.8 is "authentication credentials invalid." The SMTP server rejected your username/password (or app password/API key) when you tried to connect and send.

What 535 5.7.8 means

5.7.8 is "authentication credentials invalid." The SMTP server rejected your username/password (or app password/API key) when you tried to connect and send.

⚠️

This is a 5.x.x permanent failure — the message will keep bouncing until you fix the cause. Retrying the same message unchanged won't help.

Common causes

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Wrong username or password

Most common

The SMTP credentials in your app or device are incorrect or out of date. The most common cause.

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You need an app password, not your account password

Common

Gmail, Microsoft 365, and others require an app-specific password or OAuth when 2FA is on — your normal password is rejected.

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The account is disabled or SMTP auth is turned off

Common

Some providers disable basic SMTP auth by default (notably Microsoft 365), so it must be explicitly enabled.

How to fix 535 5.7.8

1
Re-enter the correct credentials
Double-check the SMTP username (usually the full email address) and password in your application settings.
2
Generate an app password
If your account uses 2FA, create an app-specific password (Gmail: App Passwords; Microsoft: app password or modern auth) and use that instead.
3
Enable SMTP AUTH on the mailbox
For Microsoft 365, ensure authenticated SMTP is enabled for the mailbox (it's off by default for security).

Confirm your domain is set up correctly

Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.

Authentication failed (bad SMTP credentials)