all the mails are saved on the webserver..
Not true... not true at all....
Most Email client (Thunderbird, Outlook, etc.) have in their account settings an "Offline" mode. This "Offline" mode allows you to
download the messages for viewing later without an internet connection. In thunderbird its:
Tools--> Account Settings --> (IMAP Account) --> Offline & Diskspace --> [Select folders for offline use] (button)
So in reality, you can keep your IMAP messages. The good part about imap is that it's global in that you can do one thing to it on the server, and no matter where you access it from, it looks the same to you. POP doesn't do that (although it's coming around).
so if the webserver crashed, my mails are lost..
This is exactly the reason that there are redundant backups of webservers, especially mail servers, and why you have "Offline" mode in clients. So you can still have your messages if the server crashed.