Hi, thank you for responding. Yes, it has happened with my online iCloud account. When I log in to 'Mail' in my iCloud, using Firefox, I see only the last couple of weeks-worth of emails in my Inbox, all the rest having disappeared! I can see the folders in which I've filed many other emails, but I can't see more than about 18 emails from the past two weeks in my actual Inbox. I see exactly the same on my iPhone, so I guess the issue is with Apple's iCloud Mail server? I need to know if Apple has 'lost' them permanently or if I can recover them somehow?
We do use an older Mac, running OS 10.11.06, but don't ever download emails to the Mail email client on it - we use Firefox for iCloud Mail because the older version of Safari doesn't show the emails well so we can't read them. Does that help?