This issue has nothing to do with qmail; it relates to the IMAP server, as well as to how Outlook displays dates. Lookout shows the "received" date, which, with the IMAP server versions shipping with SME <= 5.5, will be the date it was first viewed/downloaded. Other mail clients (like, for example, the webmail client that ships with SME) display the timestamp from the message's Date: header, which doesn't change.
SME 5.6, currently in development, has an updated IMAP server that doesn't have this behavior.