iphone cannot send email to exchange after os7 upgrade

I'm running Exchange 2007 SP 3, fully patched. I have a handful of users who are running OS7 on iphones 4s and 5 who are unable to send emails via exchange.

We have tried to reset network settings, delete and add accounts again, put in full email including server name but nothing is working yet. We are able to get it to work mostly via wifi connection but not via a cellular connection. Any suggestions or help greatly appreciated

thanks