I do simple DNS checks on the hosts that attempt to send mail to my server.

Wal-Mart fails:

Jan 4 23:07:18 oz postfix/smtpd[25560]: NOQUEUE: reject: RCPT from mail1.walmart.com[161.170.244.39]: 450 4.7.1 < ndc-mta1.walmart.com >: Helo command rejected: Host not found; from=< batch@ndc-fulmailapp1.walmart.com > to=< addy@ozymo.com > proto=ESMTP helo=< ndc-mta1.walmart.com >

/cs

UPDATE: I’ve been asked to provide some background on this. So here goes: My wife requested her password for her Wal-Mart online account, and didn’t receive any email. Looking at the logs, I found the above failure. I have my server configured to reject mail from servers whose hostname doesn’t map correctly via forward DNS, and thus, Wal-Mart failed. /cs