Mail App using incorrect exchange smtp address

Hi, We are currently having an issue with a lot of our companies iphone 6 users at the moment, I think this could be an exchange issue that Microsoft were looking into released a hotfix for but i don't think this has been released? The phones have been getting the issue since upgrading to IOS8. The issue we are experiencing is that the iphone's Mail App is picking up an alias smtp address attached to the user rather than their Primary. For example below is a test user that has been created on the AD with it's related smtp addresses. The iphone will pick up the address test.user@mydomaingroup.com rather than using test.user@mydomain.com which is the primary. Although the recipient still receives from the correct address it is frustrating to the user, especially when they have two mailboxes setup on the phone and when choosing one to send from they are seeing an alias.

SMTP:test.user@mydomain.com

smtp:test.user@mydomaingroup.com

smtp:test.user@mydomainplc.com

The iphone will pick up the primary address - test.user@mydomain.com when the Active Sync partnership is re-created but after a couple of days it reverts back to test.user@mydomaingroup.comagain. Does anyone have any ideas how this could be happening, or has anyone seen this before?

We are currently using Microsoft exchange server 2007

Thanks in advance

Lee Millward