This is, to put it mildly, utter bullshit.
You can store a decade of email for a million people—call it 10-20Gb each—in a 10-20Pb storage array that costs under £1M per year (and is constantly getting cheaper). 10 year old emails are *cold* storage: drives not even spinning most of the time: you only really need 5-10% of it available on demand.
The environment agency are gaslighting us. One wonders who put them up to it?
https://social.lol/@robb/115016579150112511