OK, so know I’m seeing “Could not instantiate mail function.” in the log. Which seems from "Could not instantiate mail function" error after a number of successful deliveries likely to be because it’s gone past the mail server limits.
I’m thinking that I could use:
define(‘PHPMAILERHOST’, ‘’);
$phpmailer_smtpuser = ‘smtpuser’;
$phpmailer_smtppassword = ‘smtppassword’;
Because the SMTP server seems to be working fine, it’s just using the PHP mail function that’s stuck at the moment.