Sometimes you will have a custom target that you know has a valid email address or fax number. Yet when an alert is sent to this target, the send fails and is marked as "bad email address" or "bad fax number".
If a target's email address bounces or their fax number fails several times, the target will get marked as bad. This is to prevent us from continuing to send a large volume of responses to a single mail server or fax machine that may not be designed to receive the volume.
In order to ensure that alert emails and/or faxes are processed by the target you will need to work with that target to make sure they can handle and indeed are willing to receive the messages. If they are not, then the emails and/or faxes will never go through. If the target is a part of a larger company, that company's IT department may need to get involved. Once the target is prepared to process the emails/faxes, if the emails/faxes are still showing as bad, contact support to have the targets reset.
Note that a target may work for some time and then stop working. This indicates that the server was able to and was willing to accept the alerts and then at some point stopped.