Why are emails intended for different tickets being threaded into one existing ticket?
Zendesk's mail servers will check the email headers of all emails for a message-id in the In-Reply-To or References line. If either one of these optional lines are present, Zendesk will check for existing tickets with matching message-ids. Upon finding a match, the incoming email will be threaded to the ticket.
Our hard coded logic for threading depends on emails conforming to RFC standards, meaning all In-Reply-To and References lines should contain unique values. If a static message-id or email address is used in either one of these lines, incoming emails will all be threaded into the same ticket. As a best practice, static email addresses should never be used on the optional In-Reply-To or References lines.