A tool for notifying a recipient of an email message, belonging to an email thread, that a recipient address has been added to or dropped from the thread. Address fields of the email message received are compared to the address fields of the most recent prior email message in the thread. Addressed that are determined to be added or dropped are stored in the metadata of the email message received so the added or dropped recipient addresses can be accessed to notify recipients of subsequent emails in the email thread. The metadata of previous email messages in the email thread are accessed to notify the recipient of recipient addresses added or dropped between any two consecutive email messages in the thread.