When running your second or further campaign, you may notice that your message is not sent to some of the specified email addresses. This is caused by non-delivery to those addresses during previous campaigns, which was accompanied by a serious error occurred to recipient's mail server.
For example, if you tried to send your messages to email addresses which did not exist by the time of sending, then further attempts to send anything again to those addresses will look suspicous as if you were trying to send spam to automtically generated addresses rather than useful information to your registered clients. To protect you from such suspicions, Mailer excludes once-failed addresses from the mailing list even if you manually specify them as additional emails.
Should you find out that some of the failed addresses still exist and are available for receiving mail, you may manually remove them from the failed addresses list so that your mailings are sent to them again.