Thanks to user Marc bugs with the IMAP and SMTP test modules have been identified and now resolved. These would both cause tests to falsely pass when the server or service was down.
If you are running the latest stable or similar version and don't want to update then you can simply download 1.10.3a using the link above and copy the files server/base/tests/imap.inc.php and server/base/tests/smtp.inc.php from 1.10.3a into your server/base/tests/ folder overwriting the original versions.
Bug details are on the tracker for BID251 IMAP and BID252 SMTP.