Trouble is, the “from” field is probably spoofed, doesn’t really come from david@ellesmere etc. (which may be a legitimate - and innocent - business), and could have any email address inserted there instead.
So the only safe course is never to open or download anything from an address you don’t recognise, that would have no reason to be writing to you.
Even the addresses of people you DO know are very easy to fake - if the email address book of anyone in your circle has been compromised, you could start getting malicious emails that look as if they’re from people you know!
The apparent sender may NOT be the owner of the infected computer - it could be another computer that happens to have addresses of you both!
Be particularly suspicious of emails from friends or relatives that seem oddly devoid of context, e.g. just saying something like: “Look at this!” - no niceties like hellos or how-are-yous, or anything that would personalise it to you.
If it’s a real friend or associate sending something of genuine interest, they’ll spare a line or two to tell you what it is, e.g. “Thought you might like to see pictures of my holiday.”
If you weren’t aware they’d had a holiday recently, that’s suspicious too. Check with them before opening, e.g. “I was surprised to receive holiday photos from you, as I didn’t realise you’d been anywhere! Are they definitely from you?”
They may say: “Oops, forgot to mention we were off - yes, they’re from us.” Or they may ask: “Holiday? What holiday? No, definitely fake!”
You always have to be on your guard, and look out for anything that seems out-of-character, even if the sender seems to be someone you know.
If they wouldn’t normally write: “Hi, check this out!” without further explanation (and suddenly using Americanisms), don’t trust it’s from them. It only takes two minutes to ask them: “Have you just sent me an email, only I wasn’t quite sure it was genuine?”