Windows

If it's for all messages sent to person X, to goto Person Y too then:
open up the mailbox for X under Exchange admin and goto the Delivery Options TAB. Modify the alternative recipient and select person Y from the list of mailboxes and hit OK. be sure to check the 'deliver messages to both recipient and alternative recipient' box so both X and Y will still receive the messages.

if this is a one of thing just get X to forward to Y.

If either X or Y is a mailbox not on your exchange server, but an internet mailbox you can still set them up as a custom recipient so they will show in your exchange mailboxes and you can forward to/ from them

cheers

You could set Rules in the Outlook Client on the users pc, set the rule to send copies to people in distibution list. The DL could be from the global or personal address books.
If there are multiple senders then you can set a DL from which to use the rule.
Hope this helps

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