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 [188.8.131.52] 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.