In order for our system to maintain a good reputation for sending emails we are proactive in preventing emails from being sent to email addresses which are known not to exist. Whenever an email "bounces" back from the recipient's email server that email address is automatically added to our email blacklist and the system will not send to that address for a period of 7 days. Whenever this happens you will immediately be sent an "Undeliverable" email. You will see these emails as "Bounce" or "Rejected" in the Email Log and the email address will show "BAD EMAIL" next to it throughout the system.
If you feel that an email address has been incorrectly blacklisted first check the spelling for that email address, then check the bounce message by clicking on either "Bounce" or "Rejected" in the Email Log. Bounce messages are somewhat technical, but typically include terms like "user unknown" or "this mailbox is disabled" which indicate the reason for the email not being delivered. After performing those steps if you're still sure that the email should not have bounced, contact us and we can remove it from the blacklist.