Mails rejected from valid domain

[diadem.support]

We have noticed that some mails are rejected from valid domain with proper reverse DNS. The same mail was received few days back properly from the same sender. Below is the log:

Mail not received now:

2010-10-01 13:55:05 magicspam-plesk[10647]: SPAM[check_dynamic_reverse_dns]: mua=0,ip=[66.196.100.78:web57511.mail.re1.yahoo.com],helo=<web57511.mail.re1.yahoo.com>,from=<silskies81@yahoo.com>,rcpt=<jacqueline@tcg-advisory.com>

The same mail was received earlier:

2010-09-20 11:42:43 magicspam-plesk[422]: HAM: mua=0,ip=[66.196.100.78:web57511.mail.re1.yahoo.com],helo=<web57511.mail.re1.yahoo.com>,from=<silskies81@yahoo.com>,rcpt=<bani@tcgind.com>

Can you explain why this is happening?

[wizard]

It's possible that a temporary DNS failure at the time it was 'accepted' caused this.

If it continues to happen it may be worth implementing monitoring for DNS services to identify if this is the cause.