My server is rejecting emails from one external sender. I suspect this might be misconfiguration on the sending server, but I'm not sure from these error messages.

The non-delivery report message the sender gets contains this text:

#5.7.1 smtp;550 5.7.1 Command rejected> #SMTP#

I also see this message in /var/messages at about the same time as the rejection message was sent, though I'm not sure if it's actually related:

Nov 29 12:29:28 localhost postfix/smtpd[31829]: sql_sqlite3 plugin: no result found

I'm using Plesk 10.4.4 Update #47, Centos 6.2, Postfix 2.8.4-11100615 on my mail server.

This is only happening for one sender so far, but I found a Google result on experts-exchange.com which seemed to identify the same problem and with the same sending domain. This was posted back in June, and currently has no answers, so even if I was a paying customer it wouldn't be answered. (http://www.experts-exchange.com/Software/Server_Software/Email_Servers/Q_27760746.html)

The generating server is bigfish.com.

What I need to determine is if this is a problem on my server or a problem with bigfish.com. Is there more information I can find in config files, logs, etc. to figure this out?