# Error: 535 Authentication failed: incorrect login credentials

Source: https://catalystgroup.gg/docs/email-delivery/error-535-authentication-failed/
Updated: 2026-10-19 · By Duane Garner

The SMTP server refused the username and password CRM-NG SMTP sent. Check the login, use an app password where needed, and confirm SMTP login is allowed.

> **Shown by the Other SMTP and Zoho Mail mailers (and the backup SMTP transport):** The test result or the error log shows a reply such as `535 Authentication failed`, `535 5.7.8 Username and Password not accepted` or `SMTP Error: Could not authenticate.`

 

This article is for site owners sending through an SMTP server, with the **Other SMTP** or **Zoho Mail** mailer, or through the backup SMTP transport on the **Alerts** tab.

 

## What this error means

 

Before an SMTP server accepts any mail, the client logs in. Reply code 535 means the server checked the login and said no. "Could not authenticate" is how the sending library reports the same failure. The connection itself worked; only the credentials or the login method were refused.

 

## Common causes

 
- A wrong **SMTP Username** or **SMTP Password**, including a password changed at the provider but not in CRM-NG SMTP.
- The username is not the full email address, when the server expects it to be.
- The account has two-factor authentication, so the normal password no longer works for SMTP. **Zoho Mail** and many others need an **Application-Specific Password**.
- **Zoho Mail**: the **Data Center** does not match where the account lives, so the login goes to the wrong region's server.
- SMTP login is turned off for the mailbox or for the whole organisation. Microsoft 365 disables SMTP basic login by default in many tenants.
- The account is locked after too many failed attempts.

 

## How to fix

 
1. Confirm the login at the provider
  Sign in to the mailbox's webmail with the same address and password. If that fails, reset the password there first.
2. Create an app password if needed
  For Zoho, go to Zoho Accounts, **Security**, and create an application-specific password; also make sure SMTP access is enabled for the mailbox in Zoho Mail's settings. For other providers, look for "app passwords" in the account's security settings.
3. Check the server allows SMTP login
  Ask your mail provider or admin whether authenticated SMTP is enabled for this mailbox.
4. Update CRM-NG SMTP
  Go to **CRM-NG SMTP → Settings → General**. For **Other SMTP**, make sure **Authentication** is on, then enter the full address in **SMTP Username** and the password in **SMTP Password** (press **Remove Password** first if one is stored). For **Zoho Mail**, check **Data Center**, **Mailbox Email Address** and **Application-Specific Password**. Save.
5. Send a test
  Go to **CRM-NG SMTP → Tools → Email Test**, fill in **Send To** and press **Send Email**.

 

## Verify it worked

 

The test page shows "Test email sent successfully! Check your inbox to confirm delivery." Your mailbox's Sent folder may also show the message, depending on the provider.

 

## Notes

 
- **Microsoft 365**: rather than enabling basic SMTP login, switch the **Mailer** to **365 / Outlook**, which sends through Microsoft Graph and needs no SMTP password.
- **Gmail**: use the **Google / Gmail** mailer with OAuth. If you send through Gmail's SMTP server with **Other SMTP**, you need an app password; see the 534 article.
- If **SMTP Password** cannot be changed, it is fixed in `wp-config.php`. See [secure settings with constants](https://catalystgroup.gg/docs/email-delivery/secure-settings-constants/).
- Repeated failed tests can lock the account. Wait a few minutes between attempts.

 

## Related

 
- [Other SMTP mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-other-smtp/)
- [Zoho Mail mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-zoho/)
- [Microsoft 365 mailer guide](https://catalystgroup.gg/docs/email-delivery/mailer-microsoft-365/)
- [Error: 534 application-specific password required](https://catalystgroup.gg/docs/email-delivery/error-534-app-password-required/)
- [Backup SMTP transport](https://catalystgroup.gg/docs/email-delivery/backup-smtp-transport/)
- [Troubleshooting email delivery](https://catalystgroup.gg/docs/email-delivery/troubleshooting/)