Big problem (General questions)

Hi! We are legal users of IMAP server, and while we still implementing IMAP server functionality in our system, found some big problem.
The problem is, while IMAP client fetching email-headers from IMAP folder, it also receiving sizes of the each email - body from IMAP server (at least this is behaviour of Outlook client). We have a zipped email bodies in the database, so we just creating email with header and empty body, and provide it to the IMAP server (in ListMessages event). If then client want a actual body, we get it from database, unzip and provide (in ReadMessage event). While in same session, client get it and store without problem, although the size of the body is different. But in the next session, client will get and display email with empty body! After some tests, seems it is because it getting and store exact number of bytes, whitch it was received in the ListMessages event of previous session. In our case is 0. :(
So system cant work properly in the session, other than 1st. Getting actual bodies from db, unzipping it, converting to eml to get actual size can heavily overload our server. SO:
1. Can we (by some way) to force client to get actual size of body in the next sessions?
2. If not, can you provide some way to transmit actual size of body in the ListMessages event, without assigning actual body to the message object?
Any help will be highly appreciated.
Thanx in advance.
Anton, Nordic-IT.