lammert - 1:53 am on Sep 3, 2011 (gmt 0)
You can add multiple allowed email sources by adding them on the same line. In your situation:
v=spf1 ip4:188.8.131.52/24 include:outlook.com ~all
You have to think about using ~all or -all. In the latter case receiving servers will reject all emails which are not coming from the sources mentioned in the SPF record. The ~all treats these emails as neutral. This is especially important if you sign up to forums etc with your email address, because many forums and other social sites on the net use your email address as the source address of messages they send out. With the -all setting, many of these messages will never reach their destination.