They provide pop3 access that lets you download all 18000 of those emails. You can then do whatever you want with them. Certainly set up your own webmail system

They provide pop3 access that lets you download all 18000 of those emails. You can then do whatever you want with them. Certainly set up your own webmail system on a server, say. Oh store them locally in mail-client-of-your-choice. Or import them into any other webmail service that supports importing from pop3 (none come to mind, but it would be fairly trivial to implement.)