[CentOS] Mailers and records MX

I've installed bind on centos 4.3.. This is a part of my config file:

example.com. IN MX 10 mail.example.com. example.com. IN MX 15 mail1.example.com. example.com. IN MX 20 mail2.example.com.

mail.example.com. IN A x.x.x.x mail1.example.com. IN A x.x.x.x mail2.example.com. IN A x.x.x.x

Every seems to be fine, but I'm constantly receiving messages in my 3 MX servers, even if mail.example.com (the less MX record) is available waiting for connections from outside... Why if my less mx record server is available the others servers are constantly receiving messages for my domain?

Thanks for your time Israel