I want to give this a +1, and add some further detail that may aid in troubleshooting.
We are experiencing a very similar problem. In that, a new user is created in AD (lets say usernameX1) and their email address is "usernameX1@contoso.com"
When "usernameX1@contoso.com emails our helpdesk, rather than binding it to the scanned AD user in LS (which has all the info, including assigned equipment) it will use a 'new user' style of user, without any of the AD binding. Examples included in these screenshots (personal info blocked out, obviously)
Here is the scanned AD info, which is the same username and email as the second screenshot. Notice here, there is no Helpdesk tab, and none of the user's tickets are mapped to this 'user'.

Now the second screenshot is the same user, with what looks like a LS generated amount of info, rather than binding the 2 together with the same name, email, and username:

This has only started happening recently, and is frustrating beyond belief.