It has been noticed with recent plesk releases, it creates a catch-all address called ‘trash54321@domain.com’ with the numbers being a random string. This mailbox is not visible from the Plesk interface and exists even if catch-alls are turned off for the domain. The exploit in this is that the password for this account is set the same as the username.
If someone could somehow discover the username for that catch-all account, they could log in and use the account to relay.

The issue is worth discussed in the plesk forum and i found to be useful for those  facing this issue. [URL]

I have been in search for apache clustering solution for a long time and the most professional one i could see in this regard is from IBM “Construct a highly available Apache Web server cluster that spans multiple physical or virtual Linux® servers in 5 easy steps with Linux Virtual Server and Heartbeat v2.”. Its an easy to setup tutorial using SUSE servers which i think to be useful for you if you are looking for help in apache clustering. URL