In new Scalix 12 the webmail has been improved imposing a cut-off limit to the number of messages which has to be retrieved and displayed.
By default this limit is imposed to be 1 month. That means that webmail sees there are a lot of messages but displays only the ones from the last month. That "Increase date range" has the meaning of "Widen the limit of retrieved messages if you want to see more".
If you want to change the default setting open your webmail, then access the Tools->Options menu.
In the "Receiving mail" section set the value you prefer in "Initially retrieve the following time window".