message-generate-headers-first

message-generate-headers-first is a variable defined in `message.el'.
Its value is

nil

Documentation:
Which headers should be generated before starting to compose a message.
If t, generate all required headers. This can also be a list of headers to
generate. The variables `message-required-news-headers' and
`message-required-mail-headers' specify which headers to generate.

Note that the variable `message-deletable-headers' specifies headers which
are to be deleted and then re-generated before sending, so this variable
will not have a visible effect for those headers.

You can customize this variable.