hey, i got that crap too and i can confirm that i used
http://www.makeuseof.com/tag/email-to-sms/ to setup that
yournumber@messaging.sprint.com or whatever it is.... i'm thinking its spam sent to that mailbox... not really too sure about what to do... only gotten one of those PM message things
if i start to get more i'll take off that text alert for emails and have sprint change that