Issues connecting to internal exchange 2010 email

I have come across an issue that seems to have emerged arising from the latest ios upgrade on several iphones that were working ok and now wont connect.. the backend is an internal small business 2011 server running exchange 2010 which uses its own self signed certs.. the phones were connected ok but since upgrading there is a persistent cannot verify error which kills access to the email... i have removed and readded mail accounts without joy.. i have recreated the profiles and re-downloaded the certifcates and still no joy .. i have checked the profiles are active and the trusted cert is there and checked green which they are.. everything that should work is there and enabled yet i cannot connect to the mailboxes no matter what.. i have tried this on 3 different iphones 7/8 and 10s all with the latest ios and all fail where before they didnt before.. im assuming this is to do with the certs as the issue relates to verification and authentication and my assumption is something in the latest upgrade is causing the issue but i would appreciate input or suggestions to get this resolved..

Many thanks

Ian