The purpose of a Default or Catch All account is to receive all the messages sent to nonexistent @domainname.xxx type email addresses.
Basically it catches all the errors, as it’s the default account that receives all the messages sent to nonexistent domain email accounts.

For example: if the correct name of the account is: account@domainname.xxx and, by mistake, the email is sent to: accounttt@domainname.xxx, the email will not be lost, it will be  delivered to the Default account.

If there was no Default account, the email would go back to the sender with the “nonexistent account” error message.

The Default account, can be selected by the Postmaster from the list of domain email accounts, available in the 
Management– Email Account Management menu.

ATTENTION!!! We recommend that you activate the Default option only if strictly necessary, as this type of account collects a lot of spam messages.