First, there are different ways to sort the email. By clicking on the top of any column, it will sort the column in ascending or descending order. Doing so with the "Size" column will make it easy for you to see which emails are taking up the most space.
Perhaps you might want to do another sort by the "From" column to target and delete the scanned documents that have come from Bert or Ernie photocopiers (these are often larger-size emails). Rather than deleting individual emails, holding down Ctrl and Shift, you can select several emails at a time. To select a page of emails at a time, I suggest holding down Ctrl-Shift-PgDn:
Instead of deleting emails, archiving emails allow you to retain emails without them contributing to your space quota. Notice when you right mouse click your selection the option to archive. You might wish to sort by "Date" and archive emails old emails.
Finally, your "Sent items" box also contributes to your space quota so you might wish to do the same process with emails that you've sent to others.
For easy access to Groupwise from SD28 computers and laptops, trying holding down Ctrl-Alt-G
Here are some other shortcut keys for SD28 computers/ laptops, found from the technology support page of the weebly.