Zapier only lets users with confirmed email addresses send invites to others. This means that when you receive an invite to join a team, it's always clear what the email address is of the person sending you the invitation. This way, you can determine whether or not they're someone you trust.
Zapier takes every effort to make sure you know the person sending you an invite is actually who they say they are. However, you should always make sure to confirm with your team members if you receive an email from an address you don't recognize or a general email address (using @gmail.com, @yahoo.com, @aol.com, etc.) that's not on your company domain.