[fetchmail]fetchmail + cyrus imap

[torgan]

Hello,
i got a problem with a cyrus, postfix, fetchmail configuration on my
server.
Well the story is simple and maybe the answer, too.
Iīm using following configuration and i hope i donīt forget anything.
I got postfix 2.0  installed, with cyrus imap 2 on a freebsd 4.10 system.
The simpliest thing i want to do is fetchmail from my isp mails and put
it to my cyrus imap folder.
And it works, but now comes my question:
When i do fetchmail first time my isp mailserver the mails will be
delivered to my cyrus imap folder.
So long everything is oki, but sometimes iīm a stupid boy a delete some
mails in the imap folder that after a week or so i want to read again.
I was so smart to not let fetchmail delete my mails (keep) and than
comes it the way i canīt explain to myself.
When i do a second fetchmail to the isp server the old mails iīve
already delete in the cyrus imap folder will not be saved again ???
Y that ?
Is there something i donīt get into my mind or is this the way cyrus
handles the imap folder ?  Deleted mails will never come through cyrus
to my imap folder again ???

Can somebody explain it to me or give me just a hint to an RFC where i
can find the answer ?

Torgan