[ActiveDir] AD Security permission continues to be "auto-removed"

[J B]

 We have some users that have mobile devices that connect to Exchange.  The 3rd party application uses a dedicated account to send mail from the devices.  This account needs to have "Send As..." permissions on each of the user accounts' security settings.  We have set it in all users (about two dozen) but one user in particular has a problem.  We set the permission and give it "Send As..." rights (just like all the others - no different), but usually within an hour, the newly added permission is gone - not just the "Send As" setting, but the whole account name is gone from this user's security settings as if we never added it in the first place.  We have five DC's and I have tried adding it from each DC with the same results.  I am baffled by this.  Does anyone have any suggestions?