Apparently Mayor Henry was discussing city business with council members via email, and hit the "reply all" button by mistake, sending out the email to more people than originally intended. I haven't seen the email but if Mayor Henry sent it out to all city council members, he was definitely in violation of the Illinois Open Meetings Act, which makes it illegal to call a meeting of public officals without public notification, meeting in this case defined as:
"Meeting" means any gathering, whether in person or by video or audio conference, telephone call, electronic means (such as, without limitation, electronic mail, electronic chat, and instant messaging), or other means of contemporaneous interactive communication, of a majority of a quorum of the members of a public body held for the purpose of discussing public business or, for a 5-member public body, a quorum of the members of a public body held for the purpose of discussing public business
Under this definition, sending an email out to all council members certainly qualifies as a meeting and is in violation of the Act. However, I will adhere to the old saying "Never ascribe to malice what can just as readily be ascribed to ignorance", and will assume that Mayor Henry simply forgot his duties under Illinois law. I do know he is familiar with the law as every Illinois public official, even those serving unelected on commissions, is required to complete a course on the Act and file a certificate acknowledging they did so.