Error when sending mails from API with Gmail account

I am working on an API and I am trying to send welcome mails with my Gmail account. I have configured the necessary things and it works but after a while I get this error from Postman:

Invalid greeting. response=421 Service not available: 421 Service not available

Please what do I do?

I tried to implement a solution from a similar problem but it doesn't seem to work for me, reference: Error: Invalid greeting. response=421 Service not available: 421 Service not available

Adonis Mail config file:

import env from '#start/env'
import { defineConfig, transports } from '@adonisjs/mail'

const mailConfig = defineConfig({
  default: 'smtp',
  from: {
    address: env.get('SMTP_USERNAME') as string,
    name: '[REDACTED]',
  },
  replyTo: {
    address: '[REDACTED]',
    name: '[REDACTED]',
  },

  /**
   * The mailers object can be used to configure multiple mailers
   * each using a different transport or same transport with different
   * options.
   */
  mailers: {
    smtp: transports.smtp({
      host: env.get('SMTP_HOST'),
      port: env.get('SMTP_PORT'),
      /**
       * Uncomment the auth block if your SMTP
       * server needs authentication
       */
      auth: {
        type: 'login',
        user: env.get('SMTP_USERNAME') as string,
        pass: env.get('SMTP_PASSWORD') as string,
      },
      secure: false,
      requireTLS: true,
    }),
  },
})

export default mailConfig

declare module '@adonisjs/mail/types' {
  export interface MailersList extends InferMailers<typeof mailConfig> {}
}

Gmail SMTP "421 Error Code" this means recipient server is currently unable to accept your email due to high traffic or server-side limitations, resulting in the email being deferred. Essentially, the server is indicating, “try again later.” To avoid this issue, consider using a dedicated system like Elates, AWS SNS, or Brevo.