It depends on MTA.
For Postfix
postmap -s /var/spool/postfix/plesk/virtual
For Qmail
find /var/qmail/alias/ -name ".qmail-*" -exec sh -c "echo -n {}' '; cat {}; echo" \;
If you haven't mailbox like root@domain of postmaster@domain default address will be used. Usually you can find it in /etc/aliases
Actually there is 3 layers:
1. Low level - /etc/aliases All system mailboxes forwarded to root;
2. Middle level - defaults for each MTA. How to find them - described above;
3. High level - mailboxes for domain with names of system mailboxes which were added by user.
Priority is 3>2>1 If you can't find it in 3, go to 2 and then to 1