U sUbject Optional. Developers may choose whether they wish to
generate or display subject fields.
V enVelope-to The recipient specified in incoming SMTP messages.
-W Weferences Sort of like thread-id + references + in-reply-to
- (not yet implemented. coming soon.)
+W Wefewences Previous message ID's for conversation threading. When
+ converting from RFC822 we use References: if present, or
+ In-Reply-To: otherwise.
Y carbon copY Optional, and only in Mail messages.
0 Error This field is typically never found in a message on
disk or in transit. Message scanning modules are