I'm not certain what you're looking for here, or what sort of behavior you're seeing with Mail.app here.
IMAP doesn't download mail messages. Not by default. With IMAP, the mail messages usually remain on the server unless a rule moves the messages, or if the user moves the messages manually.
The junk mail filters can also be involved here, if that mechanism is enabled.
Could you explain more about what you're seeing here?
Hamish1982 i asked the same question in another thread. i dont think that functionality of outlook express to be able to configure only the incoming or the outgoing server of an imap account is available in Mail.
AceNeerav: to avoid potential confusion...
...Microsoft Outlook Express is a rather different application from Apple Mail.app tool, beyond both being email clients. While the basic functions are similar, the user interfaces are different.
...IMAP connects the mail client to the server for listing folders and for receiving mail; IMAP is the receive path for messages, from mail server to mail client. IMAP does not provide outbound connection; that's provided by the SMTP or ESMTP server; SMTP is the send path, from mail client to mail server.
If you have a particular question about Outlook Express or Apple Mail.app tools, please link to it. (I don't see any related links in your recent profile.)