You received an Empty Segment error when testing or sending a mailing but you know there are Contacts in the Segment. This error is sometimes seen when testing a mailing too.
The most common cause of this error is due to an account setting called "cross-post merging".
The cross-post merging setting evaluates if recipients in a segment (or segments) have already received the same mailing/content within the last 24 hours. The feature determines duplicates by checking the from line, content, and segment. If they're the same as a previous recent mailing, it will prevent the mailing from going out. This setting is designed to protect you from sending to contacts who are in more than 1 segment, or sending the same mailing to the same recipients more than once. You may see a mailing in your Reports with no tracking recorded if a mailing was hit with cross post merging.