I have a domain purchased through Yahoo, and I redirected everything
to my new web & email host, with Yahoo Mail serving as a backup. I
thought, and was told, that once the name servers point to my host,
all the other settings are rendered irrelevant, and it seemed to work,
until recently. Now I'm getting about 90% of my email to my Yahoo
account, and only 10% to my host.

What's going on? Is my configuration to blame, or my host, and what
should I do to fix it?

My domain config:

Name Servers:
Primary: ns1.myhost.net
Secondary: ns2.myhost.net
Additional: yns1.yahoo.com
Additional: yns2.yahoo.com

CNAME: mail.mydomain.com -> "Yahoo! Hostname" (an internal shortcut
that Yahoo puts in)

MX:
Yahoo! Mail Server [priority 20]
Yahoo! Mail Server [priority 20]