Thanks for the hints Dan, I think I have narrowed down the problem.
The site is for a martial arts school (with different locations) which has different instructors (customers) who all place orders. When the orders are ready, they need to be collected from the main school.
I added a plugin to enable custom order status i.e ‘Ready for Collection’ but then found that I needed to enable an email notification to the customer. However Woocommerce was only letting me fix a specific email address on this status (I wish there was an option to add customers on non-default status)
That had then led me to install another plugin ‘Woo Custom Emails’ which had 2 options:
Recipients
Recipients email addresses separated with comma << I left this empty
Send Only To Customer?
( Enable this to send this email to customer. If this field is enabled then Recipients
field will be added to BCC. ) << I ticked this box.
This had been working fine in this configuration for well over a year.
BUT based on what you suggested, I could see in the header that other recipients were randomly receiving these emails, only for this status change. Indeed, when I looked closer at WP Mail Logging, it was adding people into the BCC.
It must be something to do with leaving the Recipients email addresses empty alongside changes in Woocommerce/Wordpress. For now I’ve entered another generic email address I can dump things in to to see if it patches the problem.
That particular plugin hasn’t been updated for a while so I need to hunt for something else I guess. Although as I said, I wish that I could just enable a customer notification from the in-built Woocommerce email settings screen. I’m not sure why customers can only receive emails on certain status.
Thanks again for your help
Mike