Questions

the Default Email Account catches unrouted mail - ie. other than addressed to an existing mail box.
this account’s username and password are the same as your cPanel account’s login = i.e. very precious!
be very careful with that password!
this account can’t be deleted and has no disk quota.

it is best to set this account to:
Discard the email while sending error message to the sender:
- No such person at this address -

otherwise this account will accumulate spam and take up disk space.

to configure your Default Account in cPanel go to:
Email > Email Accounts > Default Address

if that account already has email in it you might want to check there is nothing of interest there before deleting it all.