TFA says that they just deduped attachments with their homegrown system.

As I think about it more, it would make a lot of sense for a Warez group to use big storage email services with lots of accounts and some bot where you request a file and it mails you all of the compressed/encrypted chunks for you to put together. I've never seen anything like that in the wild, but it seems like it might work.

When I was younger and only had Internet access via a long-distance dial-up, Juno offered free e-mail with toll-free dial-up numbers. I spent many hours downloading files I had requested using "FTP by e-mail", which was sorta what you describe.

you just described usenet, requests or fills are usually done over irc

Usenet was a different system entirely. It was an application layer broadcast mechanism. All of the content pushed to Usenet was replicated all across the world so people could locally access it. Fantastic system for distributing current information to thousands of people worldwide over slow backbone links. Unfortunately it was also fantastic for broadcasting advertising to the entire world for free, which is why Usenet for news is dead.

Spam is very small. Totally eliminating spam will not meaningfully help your storage situation.

Yes I discovered that recently when I found my spam folder was only 200mb despite storing tens of thousands of spam. Makes sense that spam would be as small as possible for quicker delivery.

Most space in email attachments is photos and office documents.