Just a small idea. There is a problem of overflowing mailboxes. Even rising limit from 75 to 150 mails hasn't solved that. That happens mostly due to a lack of will to clean unimportant mails.
But there are also much more important causes of keeping old mails. First of all: sentimental value. I have mails I know I'll never remove from my mailbox. At any circumstances. I'm not the only one. I recall when I received a mail long time ago that was created several years ago.Second cause is: such mails that are kept within game have timestamp - they can't be forged. They are hard proofs.
Therefore I thought about such feature - Archives of Post Offices. It'd allow you to copy your mails into one long scroll and keep it on the named shelf. E.g. "archivemails 1-40 council" would store those mails in the scroll that would be named "council". "archivemails 45 council" would add 45th mail to the "council" folder etc. etc.
Archives would offer a service of copying fragments of such large scrolls to smaller pieces of paper (i.e. extracting single mails). You'd have to pay for such services (archiving and extracting). Archives would work similarly to the gems deposition system - you wouldn't have access to your archivised mails from the other POs.
I learned from discussion in the other thread that there's a problem of managing large number of small files. Let's turn them into bigger, yet fewer, files.
e.g.