It's a unique number followed by the domain jngomktg.net, which is a domain not used for anything else except the Message-ID header in emails sent by JangoMail. The Message-ID must be a syntactically valid email address, but not a real-world valid email address, as specified by RFC 2822, section 3.6.4.
In our trial and error testing, we found that the presence of the domain jngomktg.net resulted in non-delivery of the text messages to Sprint phones. Changing the domain in the Message-ID to jsmtp.net, another one of the many domains used throughout the JangoMail application, resolved the issue.
Therefore, all emails, including both broadcast and transactional emails, delivered from JangoMail to @messaging.sprintpcs.com recipients will have a Message-ID containing jsmtp.net, while all other emails will use the traditional Message-ID domain of jngomktg.net. Furthermore, we will soon be launching a feature that allows you, the user, to completely customize the Message-ID header with your own internally generated identifier.
We don't yet have an explanation as for why jsmtp.net results in delivery while the presence of jngomktg.net results in non-delivery, but we hope this information proves useful to JangoMail customers and the email marketing community at large, in resolving delivery issues to the SMS email gateway for Sprint.