do you want to send a predefined message or just have outlook connect and
send all waiting messages at a specific time?
for the first:
http://www.outlook-tips.net/archives/2004/20041230.htm
for the second: neither outlook nor windows supports it directly, but there
is (or was) at least one program that can do it - I don't know if it's
still available or recall the name.
You could also use task scheduler to open and close outlook at preset times
and if it's configured to check mail regularly, it will do a send and
receive.