Helpdesk users are required to have an e-mail because when a user creates a ticket and an agent replies, it needs an e-mail to send the reply to. Outgoing e-mail templates also use the e-mail address linked to a user to send out automatic replies.
However, it is possible to have helpdesk users without an email address. You can import users without an email field by using the import template which you can find in Configuration\User Access & Roles at the bottom of the page.
Alternatively you can create AD users without an e-mail filled in and rescan you AD users. However, do note that users without an e-mail address will generate errors when the system tries to send out e-mails to them.