public void SendMessageWithAttachment_ShouldSend_DefaultingFromAddress() { //Arrange var to = "*****@*****.**"; var cc = new List <string> { "*****@*****.**" }; var subject = "test"; var fileContent = Encoding.ASCII.GetBytes("Testing"); var fileName = "test.txt"; var message = "message"; var mimeMessage = new MimeMessage(); _mimeMessageFactoryMock .Setup(f => f.CreateFromMessageWithAttachment(_options.AdminEmail, to, cc, subject, fileContent, fileName, message, "")) .Returns(mimeMessage).Verifiable(); //Act _service.SendMessageWithAttachment(to, cc, subject, fileContent, fileName, message); //Assets _mimeMessageFactoryMock.Verify(); _mimeKitServiceMock.Verify(k => k.SendEmail(mimeMessage)); }