Gravity SMTP Failing

One of our sites stops using the configured SMTP gateway in Gravity SMTP and it also stops logging outgoing emails when the Clean Talk plugin is activated.

According to the logs, wp_mail() works correctly, but PHPMailer is being overridden, which is why Gravity SMTP cannot pass the email through its own SMTP layer.

Once CleanTalk is deactivated it starts working again. Any ideas?