Failed email - Logfile explanation needed

[MustangPriMe]

We're having some email failures which go largely unnoticed as there's no bounce-back or NDR generated. It's only come to light from someone expecting a particular email.

The ISP of the email recipient have supplied this portion of their log (IP and domains altered).

Can someone shed light as to what is happening here, and whether the fault lies with the sender or recipient's ISPs?

Thanks
Log follows:

Apr 10 14:40:00 mail4 postfix/smtpd[5501]: NOQUEUE: discard: RCPT from sender.smtp.mail.com [111.111.111.111]:
sender.smtp.mail.com[111.111.111.111]>: Client host triggers DISCARD action;
from=<sender@zzz.com>
to=<recipient@zzz.com> proto=ESMTP
helo=<sender.smtp.mail.com>

 

[eyec]

spam/junk filter on the receiver's end would be my first guess.

 

[smah]

I agree, for some reason the server is just dropping it based on the sender's host name. My guess is that the sending mail server is set up to be an open relay or spam is being sent through that server because of a zombie machine.