With regards to this, the odd one or two, even quite a few spread over your duty there is within reason not a lot you can do.
But...
If a mass lot of new builds start to get built and you notice they all have either dodgy or low level letterboxes, then get on to the business straight away, your rep, H&S rep etc as soon as possible.
You could get the doors changed, it has happened before my H&S area rep told me once.
If too late, then a risk assessment would need to be done and let's be honest, it's not going to go down too well if you constantly have to bend down to post the mail.
It can be done if there are enough problems and it's a danger to your H&S.
Don't just suck it up!
I Wrote-During Covid-Which is still relevant now
It's good to get these types of threads, the ridiculous my manager said bollox, so we can reassure ourselves that while the world is falling apart, Royal Mail managers are still being the low-life C***S they have always been. My BFF Clash The daily grind of having to argue your case with an intellectual pigmy of a line manager is physically and emotionally draining.