OT Verizon rejecting emails from gmail

I don't know who else to ask, but yesterday and today I've had trouble
sending email to a friend who has a verizon.net address. Verizon keeps
rejecting them. Here's what I get back from them. I just don't know
who else to ask. Has anyone else had this trouble?

thanks

Message-id:<4FF90584.6030107@gmail.com>
Date: Sat, 07 Jul 2012 20:59:00 -0700
From: Derek<######@gmail.com>
To: John<######@verizon.net>
Subject: just checking

Your message cannot be delivered to the following recipients:

Recipient address: @relay.verizon.yahoo.com:######@verizon.net
Original address:######@verizon.net
Reason: SMTP transmission failure has occurred
Diagnostic code: smtp;554 Message not allowed - [320]
Remote system: dns;mx1-vzn.mail.yahoo.com (TCP|206.46.172.79|34174|67.195.168.231|25) (mta1012.vzn.mail.ac4.yahoo.com ESMTP YSmtp service ready)


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