Internet Message Format (IMF) is specified by RFC 5322. It defines a variety of header fields, such as To
, From
, and Subject
.
It also defines the Comments
header field and the Keywords
header field.
https://www.rfc-editor.org/rfc/rfc5322#section-3.6.5
Does anything actually use these two header fields today, or are they totally superfulous? In other words, are mail clients in modern use expected to make use of these?