Using the Mail Forwarding Settings found in the Edit Mailbox screen create two problems: bad entries in /etc/alias, and a mail forwarding loop if delivering to another user in same domain.
(Note: this is experienced on a system configured to use user@domain username format)
In Mail Forwarding Settings for user test1@testing.com, I check the boxes for "Deliver to this user normally" and "Forward to other addresses" and then put test2@testing.com in the box.
Virtualmin creates the following line in /etc/aliases:
test1-testing.com: test2@testing.com,\test1@testing.com
First problem is the spurious backslash (though it seems that Postfix ignores it). Second problem is the use of test1@testing.com in the alias to provide local delivery -- that creates a mail loop because of the following entry in /etc/postfix/virtual:
test1@testing.com test1-testing.com
And 'round and 'round we go. :)