From the Spiceworks forum:

# get all mailboxes that have a forward, sort by name and select their name and forward address 
$fwds = get-mailbox | Where-Object { $_.ForwardingAddress -ne $null } | sort Name | select Name, ForwardingAddress

# now get the primary smtp adress of each forward address 
foreach ($fwd in $fwds) { 
$fwd | add-member -membertype noteproperty -name "ContactAddress" -value (get-Recipient $fwd.ForwardingAddress).PrimarySmtpAddress 
}

# finally excport to a CSV without the annoying type header 
$fwds | Export-Csv c:\forwards.csv -NoTypeInformation