By default webmail provides thread support. This can be disabled, or modified in the "settings" tab of webmail. Thread support using it's default values will check only the last 6 months of e-mails, so any e-mails that were received over 6 months ago would no longer be displayed.
In the "setting" tab of webmail you can either disable thread support completely, or you can change how old an e-mail webmail will still display.
