MXレコードには優先度というものがありますね、こんなん
example.com 3600 IN MX 5 mail1.example.com example.com 3600 IN MX 5 mail2.example.com example.com 3600 IN MX 10 mail3.example.com example.com 3600 IN MX 10 mail4.example.com
ブラックリストとかやらかして、全てのメールサーバーにBANとかされていると、メールログには
relay=mail3.example.com relay=mail4.example.com
しか出てこなくなります。
これは別に優先度の低いやつにしかアクセスしていないのではなく、最後にアクセスしたやつのログが残っているだけのようです。
バウンスメールを確認すると、
... while talking to mail1.example.com.: <<< 554-mail1.example.com <<< 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means. ... while talking to mail2.example.com.: <<< 554-mail2.example.com <<< 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means. ... while talking to mail3.example.com.: <<< 554-mail3.example.com <<< 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means. ... while talking to mail4.example.com.: <<< 554-mail4.example.com <<< 554 Your access to this mail system has been rejected due to the sending MTA's poor reputation. If you believe that this failure is in error, please contact the intended recipient via alternate means.
こんなんでていると思います。