Suppose you would like to send a copy of an incoming email to another account while the email still goes to the original account. You can do this by modifying the .qmail file of the original recipient and adding an entry that looks like the following:

&user@domain.com

On a Plesk server, a full .qmail file with the above changes looks like this:

| true
&user@domain.com
| /usr/bin/deliverquota ./Maildir

Keep in mind that if you do this on a Plesk server, these changes will be overwritten with mchk or upgrades so you will have to take measures to deal with this. I set the files immutable so they can't be changed by Plesk. This causes its own problems so you will have to decide if it is worth it.