"Content-length:". MIME formats are chosen and/or converted based on the
client's preferred format settings, which are set using the MSGP command,
described below.
+
+ The MSG4 command also accepts an optional second argument, which may be the
+MIME part specifier of an encapsulated message/rfc822 message. This is useful
+for fetching the encapsulated message instead of the top-level message, for
+example, when someone has forwarded a message as an attachment. Note that the
+only way for the client to know the part specifier is to fetch the top-level
+message and then look for attachments of type message/rfc822, and then call
+MSG4 again with that part specifier.
+