E-Mail mailbox passwords are completely self-configurable from your MyPANEL, so if you cannot access a mailbox because you cannot remember the password, or need to change the password for any other reason, just access your MyPANEL then go to:

'Email Settings' -
'Main Settings' submenu-
Go to 'POP3 Mailboxes' and click on the mailbox (E-Mail address) you need to change:
Enter your new password choice in both fields in the 'Choose a new password (type once in each box)' section.
Click 'Change'

You will then receive a 'Your password has been successfully changed' message and can use the password to access the mailbox in question.