In the recent release of the expansion of Quantum Rise, CCP have announced that in the latest change the in-game email system have been changed, let's move on to what has been changed here.
CCP Wrangler says: "To address performance issues in EVE we are making some changes to the EVE Mail system in Quantum Rise 1.0.1. You will now be able to fetch up to 1000 undeleted EVE Mails, however EVE Mail number 1001 and beyond will not be fetched, whether it has been read or not. To gain access to those EVE Mails, you must delete newer EVE mails. In addition to this all EVE Mails sent by NPCs that are older than 3 months will be deleted."
Then players in the related forum have raised their voice that there is a kind of sentimental value attached to the kill mails which were received upon a ship destruction since the early days of EVE, whenever a player managed to kill another and so on...
The announced change to EVE Mail in Quantum Rise 1.0.1 doesn't necessarily state that those messages will be deleted, but if they're inaccessible, they may as well be, at least according to some of the more vocal EVE players on the forums.