You're absolutely right. It's definitely a tricky one to diagnose.
I actually already have every email trigger set to send a copy to a dedicated archive email address which I monitor but nothing had actually come through which is why I thought it odd that I had supposedly exceeded my send limit.
I'm going to continue to monitor it anyway, and introduce your authenticated email option. I think I may also set up a simple page triggered via cron-job.org which sends an email to me once a day at the same time, and then at least I know if I don't receive an email, then another block is in place.
Thanks again for your support.