I know I cannot delete a mailbox once it's created, what can I do if I've already created one but do not need it presently?
You can reuse the mailbox later. You can set the mailbox with "dummy" information which you would replace with the new mailbox information when it's time. Make sure that the field "Enable Mail Retrieving" is unchecked. NOTE: If you are setting more than one mailbox for reuse, add a sequential number after the login, e.g. "changeme1", "changeme2".
See the screenshot below: