Your "default address" is the root catch-all mailbox. It receives messages heading to all addresses which aren't setup as unique pop accounts within your domain. Keep in mind that the default address account does NOT use the full email address format, unlike other pop accounts. The login for your default address is the same as the main FTP login for your account.

In our attempt to stop SPAM from filling up your default mailbox, your hosting account is automatically set up to reject emails intended for non-existing mailboxes. If you enable the default mailbox, please make sure to clean it up periodically in order not to avoid mails being rejected due to your defaullt mailbox being over-quota.