I did not, I hadn't seen that - thank you for pointing me in that direction. I've submitted the request and will update here if that resolves once actioned.
Plesk Obsidian 18.0.69
Lightsail VPS
Stock install, with licence - no extensions added
I have added 3 domains and enabled webmail on all 3. Websites all resolve fine. Incoming mail arrives fine
However, anything trying to be sent from them is not being delivered. Looking at the mail queue...