Is an ESMTP server required to send txt msg via email?

[dhesford]

Working on an app that sends txt msgs via a hosting provider's mail::sendmail server.  I can successfully send messages to other mobile phone providers, but about 90% VzW msgs fail.  Occasionally a msg is delivered though usually delayed by hours.  I have validated my DKIM, SPF & _Demarc DNS records.  

The failed emails report the following:

    host smtpin01.vzw.a.cloudfilter.net [50.16.175.197]
    SMTP error from remote mail server after initial connection:
    554 vzw-ibgw-5001a.stratus.cloudmark.com cmsmtp ESMTP server not available - POL143 - NMIvhnZJcL68J

Don't understand why an ESMTP would be required since there are no attachments.  The emails contain 100 or less characters in plain text.  This is a commericial system in development, but reliable txt msg delivery is paramount before moving its pilot phase.