An SMTP authentication error means the sending client could not complete the required sign-in or authorization. It is different from a network timeout or a recipient rejection. Confirm the account and supported sign-in method before changing passwords or server ports.
Prefer the client’s Google sign-in flow
Update the mail client through its official channel and use its Google account option or Sign in with Google where supported. Complete Google’s authorization and any required verification in the genuine Google flow.
A normal account password pasted into an older basic-authentication dialog is not equivalent to OAuth authorization. Do not turn on obsolete less-secure-app access or disable 2-Step Verification to make the old dialog work.
Check whether an app password is appropriate
Google recommends modern sign-in and says app passwords are unnecessary in most cases. They require eligible accounts with 2-Step Verification and may be unavailable because of account type, policy, security-key-only setup, or Advanced Protection.
If an approved older device genuinely requires one and the account permits it, use the official app-password process. A Google password change revokes existing app passwords, so an old device may need a replacement. Treat the generated secret like a credential and never include it in support screenshots.
Separate account policy from transport settings
- Verify the full account address used as the username.
- Check the provider’s correct SMTP host and encrypted connection method.
- Review the exact authentication error and recent account changes.
- Ask the administrator about restrictions on managed accounts or devices.
For a Google-hosted SMTP route, Google’s device guidance documents smtp.gmail.com and secure connection choices, but relay and other provider configurations are separate. Do not copy an unrelated server’s settings into every client.
Preserve unsent mail before removing or re-adding the account. This article addresses the sign-in distinction; existing client-specific setup guides should supply the appropriate screen-by-screen configuration. Use one harmless test after a justified correction rather than repeatedly submitting private mail with uncertain credentials.
Browse Gmail & Google Account help
References
- Add Gmail to another email client
- Sign in with app passwords
- Send email from a printer, scanner, or app | Gmail | Google Workspace Help
Reviewed against the linked product guidance, standards, and identified vendor or public-authority guidance on October 29, 2026. Available options and screen labels can vary by account, device, region, and organization policy.