The User-Agent: header can be fun and interesting, but it also leaks
quite a bit of information about the user and their software stack.
This represents a potential security risk (attackers can target the
particular stack) and also an anonymity risk (a user trying to
preserve their anonymity by sending mail from a non-associated account
might reveal quite a lot of information if their choice of mail user
agent is exposed).
This change also avoids hiding the User-Agent header by default, so
that people who decide they want to send it will at least see it (and
can edit it if they want to) before sending.