535-5.7.8 Username and Password not accepted

Time flies, the way I do without enabling less secured app is making a password for specific app

Step one: enable 2FA

• https://myaccount.google.com/signinoptions/two-step-verification/enroll-welcome

Step two: create an app-specific password

• https://myaccount.google.com/apppasswords

After this, put the sixteen digits password to the settings and reload the app, enjoy!

  config.action_mailer.smtp_settings = {
    ...
    password: 'HERE', # <---
    authentication: 'plain',
    enable_starttls_auto: true
  }

I had the same problem. Now its working fine after doing below changes.

https://www.google.com/settings/security/lesssecureapps

You should change the "Access for less secure apps" to Enabled (it was enabled, I changed to disabled and than back to enabled). After a while I could send email.

Google stopped allowing apps logging in to Gmail using real password

After 30 May 2022 you can't login with username and password alone to Gmail. Less secure app access is not available anymore unless you have Google Workspace or Google Cloud Identity.

Source: https://support.google.com/accounts/answer/6010255

You need to make a password for specific app

Step one: enable 2FA

• https://myaccount.google.com/signinoptions/two-step-verification/enroll-welcome

Step two: create an app-specific password

• https://myaccount.google.com/apppasswords

After this, use that 16 digit

First, You need to use a valid Gmail account with your credentials.

Second, In my app I don't use TLS auto, try without this line:

config.action_mailer.smtp_settings = {
  address:              'smtp.gmail.com',
  port:                 587,
  domain:               'gmail.com',
  user_name:            '[email protected]',
  password:             'YOUR_PASSWORD',
  authentication:       'plain'
  # enable_starttls_auto: true
  # ^ ^ remove this option ^ ^
}

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UPDATE: (See answer below for details) now you need to enable "less secure apps" on your Google Account

https://myaccount.google.com/lesssecureapps?pli=1