synchronize sent-mail over several machines?

[Zero Void <hujingcao@hotmail.com>]

Here is what I want to achieve:

I ususally use several machines, one in my office and one at home, etc. And
I also use an imap mail box. And what I want to do is to synchronize the
email that I sent so that all of the emails that I sent will appear on all
those machines, _locally_, sooner or later. (I don't want to store all my
sent-mail to my imap server, as the space is somehow limited).

So I guess one way to do that is:
1. Set the Gcc so that every time I sent an email, a copy will be sent to
imap:INBOX.sent-mail.
2. in .gnus file of each machine, set up nnimap split so that all the files
in INBOX.sent-mail will be copied to a local folder. 
3. expire all the emails in INBOX.sent-mail, no matter read or unread, after
say 30 days. So it's very likely that during the period, all the machine
will have a copy of all those sent-mails. 

If this is a correct way to go, then my question is: 
1. How can I keep a copy in INBOX.sent-mail after it has been
splitted? (Since I need to do that on different machines with the same
set of emails). Also, as I have already used nnimap-split-inbox for my
INBOX mails, I am not sure whether I can split to nnimap mail folder
using different policies. 
2. How to automatic expire all the emails no matter it's read or
unread?

TIA.




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