For more information on your device, select it on the Support main page.
Depending on how you have created mailboxes perform the following:
Open Menu > Messaging > Options > Settings > E-mail > Mailboxes and then long press on mailbox opens the selection to delete it.
Or select Menu > Applications > Email > Options > Tools > Settings > Highlight the mailbox you are deleting > Options > Remove mailbox.
Applies for Nokia C5-03, C5-05 and C5-06 phones.