Why Am I Getting SMTP Authentication Error 535: Username and Password Not Accepted with Gmail?

SMTP Authentication Error 535: Username and Password Not Accepted with Gmail

When attempting to send emails from a localhost using the provided function, users may encounter the following error:

smtp error: 535 5.7.8 Username and Password not accepted for gmail...

Despite ensuring valid credentials and enabling "Allow less secure apps," this issue can still occur.

Cause

The error indicates that the authentication credentials used are incorrect or not authorized. Google has recently implemented stricter security measures, which can prevent email sending from certain apps.

Solution

To resolve this issue, it is recommended to:

1. Enable 2-Factor Authentication (2FA)

• Navigate to https://myaccount.google.com/security and enable 2FA if not already.

2. Generate an App Password

• Visit https://security.google.com/settings/security/apppasswords
• Select the "Mail" option for the device and application
• Generate a 16-digit App Password

Using the App Password

Replace the regular password in the SMTP authentication code with the generated App Password. The updated code will look like this:

err = smtp.SendMail("smtp.gmail.com:587",
        smtp.PlainAuth("", from, appPassword, "smtp.gmail.com"),
        from, []string{to}, []byte(msg))

Note: An App Password provides restricted access to your Google Account specific to the app or device.

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