These tests are not trying to send email, the part of the code that's failing is the input validation which uses Email::Valid with the MX check enabled.
I found your problem: the example.com domain is not really even supposed to resolve, much less have ever had valid MX records. The validation is working correctly: email addresses on example.com are not valid and you should not be using those in test data that is supposed to pass validation.
I offer a solution: establish a ci-test subdomain on your own domain and use it for your sample customer records.
