It should be easy enough in Outlook, but I know for a fact that this is dead easy in
Thunderbird and recommend it over any Outlook variation at any time.
If you use the calendar at all, then it's easy to get a calendar extension for Thunderbird, as well as other things that make it the best email client that no money is needed for.
Still a web based application??? I don't think that would be a good idea at all, since somebody might do you a nasty "favour" and break into it.
Some programming guru could probably cook up something like that, but to my knowledge no such easy solution exists.