Password Reset email not going via WP Mail SMTP

I’m running a WP Multisite server, where one of the hosted domains is as per the link above. I only want to run WP Mail SMTP on this one domain, so installed the plugin then only activated it on this domain.

I’ve set up with GSuite relay and it works fine from the test form.

However, if I go to the password reset option on the site for any user, it still sends the email from the built-in PHP function (and coming from the canonical host rather than the domain for which I’ve configured WP Mail SMTP). So how do I ensure that all domains from a site come from that domain and via WP Mail SMTP?

thanks
Richard

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