For some reason, Salesforce is adding a sandbox name at the end of all contact's email address'. So for example if my contact's email is firstname.lastname@example.org and it ends with email@example.com. Any ideas why it happens?
This is standard behaviour to ensure that no unwanted emails actually reach the person. You may have automation in your org that spams the recipients, you probably don't want this to happen uncontrolled. So by invalidating email addresses (and making sure you can find out what is going wrong) Salesforce prevents you from automatically and involuntarily sending out emails.