Tracking software permits the sender of an email message to secretly monitor the receipt and subsequent handling of that message, including any attachments. ISBA Opinion No. 18-01 (January 2018) cautions that the use of tracking software by lawyers in emails or other electronic communications with other lawyers or with clients is only permissible if the recipient gives "informed consent" to the use of the tracking software. If used without the informed consent of the recipient, the secret monitoring of a recipient's handling of the lawyer's email constitutes dishonesty and deceit, and if sent to another lawyer in the course of representing a client, the sending lawyer could be covertly invading the confidential relationship between the receiving lawyer and that lawyer's client. Comment [1] to Rule 4.4(a) explains that unwarranted intrusions into privileged relationships, such as that between attorney and client, could sometimes be considered obtaining evidence using methods that violate the rights of a third person. (The Opinion notes that any email seeking consent to the use of tracking software must itself be free of tracking software, contain no other substantive content, and give the recipient a clear, explicit, and non-technical plain language explanation of the software that the lawyer proposes to use and that the receiving lawyer should not consent without first obtaining the informed consent of his or her own client.)