If you enter an invalid e-mail address or unknown user or user group name when inviting people to collaborate on a model package or project, it is indicated immediately. Check the address or name and correct it.
When entering an invalid e-mail address or unknown name in Enterprise Studio, the person's initials to the left of the e-mail box are not shown. Instead, an exclamation mark is shown in red:
When entering an invalid e-mail address or unknown name in the Team Server, the borders of the box are displayed in red: