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