Error when Cron runs

I'm getting this error when the cron runs on email, an ideas where to start to correct this issue?

Unhandled System_Core_Exception: Inbox error: tpsmg-mail.hspheredns.com ESMTP in /hsphere/local/home/tpsmg/your-purl.com/system/mail/inboxengine.inc.php on line 297
Stack trace:
#0 /hsphere/local/home/tpsmg/your-purl.com/system/mail/inboxengine.inc.php(73): System_Mail_InboxEngine->handleError()
#1 /hsphere/local/home/tpsmg/your-purl.com/system/mail/inboxengine.inc.php(45): System_Mail_InboxEngine->setSettings(Array)
#2 /hsphere/local/home/tpsmg/your-purl.com/cron/job.php(328): System_Mail_InboxEngine->loadSettings()
#3 /hsphere/local/home/tpsmg/your-purl.com/cron/job.php(76): Cron_Job->processInboxEmails()
#4 /hsphere/local/home/tpsmg/your-purl.com/system/core/application.inc.php(210): Cron_Job->execute()
#5 /hsphere/local/home/tpsmg/your-purl.com/system/bootstrap.inc.php(130): System_Core_Application->run()
#6 /hsphere/local/home/tpsmg/your-purl.com/cron/job.php(690): System_Bootstrap::run('Cron_Job')
#7 {main}