It's called threading, and if you look around in the options you'll probably find a setting to turn it on/off.
Just do take care using Outlook. It's a spammer's wet dream of an email client. Low security, easy to exploit default settings, uses Internet Explorer to render all email messages, etc. You'd be wise to find an alternative, but if you insist on continuing to use it for whatever reason, just be sure to be very careful.
Lately outlook express is clustering my mails
I thought it was only when I have long discussions going out and in with the same person, but sometimes it merges insides massages from other people
My inbox is decorated with small squares and a cross inside them, when a new mail is coming the line become bold again and to read the new massage I have to click on the square
This is all very annoying how do I get rid of it
With Christmas spirit- please help me