The xav.com domain has a big spam problem -- it receives several thousand spam messages each day, and only about a couple dozen legitimate emails.
Various approaches have been tried to get around this problem. Content-filters were tried but they would only block about half the spam. Also, they'd often block a legitimate message by mistake. Later an elaborate challenge-response system was set up, but it failed because the underlying software couldn't handle the volume of incoming mail.
The current solution has been to shut down all previously-used email addresses. A new set of email addresses has been created, for use on the Contact page or in the Reply-To field of outgoing emails. Those new email addresses use temporary names that will change over time as they start to show up on spammers' lists.
The upshot of all of this is that if you try to email Fluid Dynamics using any address more than a year old, it will bounce. That is unfortunate but there is no way around it. The other consequence is that you no longer need to include whitelist strings in your email, like "zm_no_delete" or "XAV62". There is no longer any whitelisting or filtering going on.
"Spam filtering" http://www.xav.com/scripts/misc/1014.html