Over the past few days I have been playing about with the DNS on my LAN (changing which server runs dnsmasq, etc.).
Last night around 22:00 I left everything, as I thought, running OK. This morning though no E-Mails had arrived on my desktop (which is also the SMTP server). It turned out that I hadn't restarted things enough and my dekstop had got a randomly assigned IP address rather than the fixed one it should have got (by telling dnsmasq its MAC address).
So TsoHost's mail system hadn't been able to deliver mail to zbmc.eu for something like 12 hours.
It was a good test for two things:-
My offsite mail backup to which I deliver all my mail in parallel to delivery to my dekstop worked perfectly, so I could see all the undelivered mail there.
When I fixed the IP address of my desktop all the undelivered mail arrived on my desktop after an hour or two.
Isn't it nice when things work as they should! :-)