The members of the iesg-only mailing list (firstname.lastname@example.org) are all and only the current Area Directors (and for a brief period each year also the incoming Area Directors), plus an email address that is subscribed for archiving purposes (the list is archived at a password-protected area of the ietf.org website, accessible only to ADs).
Note that the email@example.com mailing list contains, next to the Area Directors, the Secretariat, IANA, the RFC Editor, the IAB Chair, and the liaison from the IAB.
As currently configured, the iesg-only mailing list discards email from most senders, except for emails from ADs themselves (the mailing list members) plus a few people who are explicitly whitelisted for sending to the list.
In order to keep IESG discussions transparent to the IETF community, ADs SHOULD use the firstname.lastname@example.org list instead of the iesg-only list, except as described below.
The iesg-only list MUST be used instead of email@example.com for messages that require confidentiality - for example messages involving personnel issues or messages involving disputes with another SDO. Of course, messages from !NomCom that include information about nominations and nominees for confirmation by the IESG also require confidentiality, but those discussions take place on firstname.lastname@example.org.
The iesg-only list SHOULD be used for sensitive personnel-related discussions (see also IESG Internal) and MAY be used for soliciting advice on WG management problems.
The iesg-only list MAY also be used for coordination on contentious issues involving other SDOs.
The iesg-only list MAY be used for exchanging silly jokes, but keep in mind that they can be seen in the archive by future IESG members