Hi Joachim,
Unfortunately, there is no way to determine whether the test execution was stopped with the Stop button.
As far as I understand, you create the OnStopTest event handler to send e-mails when the test is stopped. In this case, TestComplete sends an e-mail if the test is stopped manually as well.
As a workaround, you should not to use the OnStopTest event. You should add a function that sends e-mails at the end of each test. In this case, e-mails will be sent when the test is over, and they will not be sent when you stop the test manually.