On a Plesk server on Google Cloud, sending email is configured using a G Suite (former Google Apps) accountю
However, messages are bounced by G Suite with the following error:
host smtp-relay.gmail.com[220.127.116.11] 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
Misconfiguration of the G Suite account.
Re-check the G Suite settings as per the following article.
For any assistance with configuring G Suite account, contact support as described on this page.