You received an Empty Segment error when testing or sending a mailing but you know there are Contacts in the Segment.
The most common cause of this error is due to an account setting called "cross-post merging." What does THAT mean?
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. 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.