Mail to wildcard MX records doesn't work from Yahoo Mail, but fine from other addresses

[Bob Eastbrook]

I use wildcard MX records for mail, and a wildcard CNAME for web
traffic.  For example:

     *.example.com = MX record for mail.example.com
     *.example.com = CNAME myapp.appspot.com

Email to b...@foo.example.com gets delivered to mail.example.com, and
web traffic to http://foo.example.com goes to myapp.appspot.com.  I
use instructions from Wietse from a post I made on Dec 31, 2009:
http://www.pubbs.net/200912/postfix/75444-virtual-domains-for-wildcard-mx-records.html.

This works for all mailers I've found except for Yahoo Mail.  Mail
sent from Yahoo is rejected with:

     <b...@foo.example.com>:
     [ip.number.of.mailserver] does not like recipient.
     Remote host said: 554 5.7.1 <b...@myapp.appspot.com>: Relay access denied
     Giving up on [ip.number.of.mailserver].

At first glance, it appears that Yahoo Mail ignores the wildcard MX
record and tries delivering to the CNAME.  This is puzzling because my
mail server also handles *.example.org in the same fashion as
*.example.com, and example.org addresses work fine from Yahoo.  Note
that my mailserver is mail.example.com, not mail.example.org.

Here's what I have for mydestination:

     mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain

I run postfix 2.3.3 on CentOS 5.4.

Any ideas?

Bob