When you buy a new home or rental property does the law require you to redirect mail received for earlier owners/tenants?

I hardly send snail mail these days but receive mail for others who lived at my address recently or even several years ago. I don't mind forwarding on the odd letter saying "Not at this address" but when there is mail almost every day what must one do with it all?

When the previous tenants have left no forwarding address, perhaps because they want their privacy, can I bin it, must I hang on to it for ever, or am obliged to redirect it either to the letting agency that rented out the property, or write "No forwarding address" or "Owner no longer at this address - Return Sender" or some such and make a unplanned trip to a Post Box to send it on.

If I have to forward/redirect mail, how many weeks, months, or years am I required by law (if any) to do this? Do I need to forward it within a fixed number of days of receiving it or perhaps once a month?

Others must have this hassle too. I presume it would be illegal to open it even if there is no return address on the outside of the letter and perhaps also illegal just to throw it away.