Use Exchange console...

This example creates a mail contact for John Doe.

New-MailContact -Name "John Doe" -ExternalEmailAddress This e-mail address is being protected from spambots. You need JavaScript enabled to view it -OrganizationalUnit akusz.com

This example mail-enables an existing contact named John Doe.

Enable-MailContact -Identity "John Doe" -ExternalEmailAddress This e-mail address is being protected from spambots. You need JavaScript enabled to view it