postix, cyrus imap and online backup

[Boris Glawe]

Hello,

we're using postfix and cyrus imap on the same machine for internal 
mail. Mail delivered by smtp to the machine is locally forwarded to the 
imap server.

Currently all mail is handled by a domain provider. We fetch our 
(external) mails via POP3 from the provider's server.


We'd like to extend the usage of this mailsystem in a way, that it will
1.) send all external mail,
2.) receive all external mail
*and*
3.) do an online backup of all mails that have been transferred.


1.) is easy to solve.
2.) What's the best solution here?
    - we could forward mails from our domain provider to our
      postfix machine. What happens if that machine isn't online
      for a few days? Is there a solution that the mail is
      regularly being fetched (every 5 minutes) from the
      provider's server?
    - what else do you suggest?
3.) The backup can of course be done by syncing the imapd's directory 
regularly to another machine. Problem is that the backuped data 
structure is an cyrus specific data stucture that can hardly be used to 
restore lost mails. We'd like to have a solution that makes it easy to 
restore lost mails and - if possible - is an online backup solution. 
That means each mail the is delivered or received will automatically be 
backuped while it's being handled.

Thanks for any suggestions.
If you have suggestions to use a different software environment for mail 
delivery, please tell me.

Thanks and greets

Boris