Unable to send mail through G Suite from a Plesk server on Google Cloud: Invalid credentials for relay

Symptoms

On a Plesk server on Google Cloud, sending email is configured using a G Suite (former Google Apps) account as described in https://support.google.com/a/answer/2956491 .

However, messages are bounced by G Suite with the following error:

CONFIG_TEXT: host smtp-relay.gmail.com[74.125.200.28] said: 550-5.7.1 Invalid credentials for relay [203.0.113.2]. The IP address you've registered in your G Suite SMTP Relay service doesn't match domain of the account this email is being sent from. If you are trying to relay mail from a domain that isn't registered under your G Suite account or has empty envelope-from, you must configure your mail server either to use SMTP AUTH to identify the sending domain or to present one of your domain names in the HELO or EHLO command

Cause

Misconfiguration of the G Suite account.

Resolution

For assistance with configuring G Suite account, contact their support as described here: https://support.google.com/a/answer/1047213 .

Additional information

How to send email from a Plesk server on Google Cloud?

How to configure Plesk to use G Suite