In order to have all messages sent to a user (we will user email@example.com) redirected to a specific address (we will use firstname.lastname@example.org), you must take the following steps:
1) Via Webadmin -> Security & Filtering -> Incoming Message Rules
2) Click on "Add Message Rule"
3) Type a name for this rule
4) In the "Matches" area select:
To - Contains or Matches - type the email address to match; in our case email@example.com
5) In the "Actions" area select:
Redirect To - type the email address to redirect to; in our case firstname.lastname@example.org