The To, Cc, Reply-To, and Subject headers are optional (according to RFC
2822, section 3.6). They can occur zero or 1 times within the sender's
*data*. Since this data is whatever the sender wants to include when
using the DATA command to their SMTP server, the sender doesn't even
have to include these headers, the To and From fields might not even
contain e-mail addresses, and the Subject header, if it exists can have
a blank value or anything in it. E-mail clients that force the user to
use a non-blank Subject header are violating this RFC by enforcing a
header that is optional.
You are confusing Outlook with Outlook Express. Outlook is a component
of Microsoft's Office suite. Outlook Express is a component of Internet
Explorer. Outlook is oriented to a business market. Outlook Express is
oriented to personal use. Outlook and Outlook Express are not related,
they are not siblings of each other, they are not code derivatives of
each other, and they are not a "full" and "lite" version of the same
program. Outlook Express will prompt you if the Subject header is
empty, but also gives you the option to quiesce this prompt. Outlook
won't prompt you.
