I've (mostly) switched from Outlook to Thunderbird for mail. If you, like me, subscribe to a few mailing lists, you know it's a pain dealing with them. If I don't have time to read the issue from 6 months ago, it's nice to have it just disappear. No stress trying to decide whether to do a quick scan or just do a bulk delete. Just delete them.
One of the features I've missed with Thunderbird was that ability to delete messages automatically. No stress. Well, unless you delete the wrong messages. But Thunderbird has good filters and so it's easy to avoid that.
Message aging explains how to turn on message aging (as in delete all messages past X days old or delete all messages so there's just X in the folder) for all of your mail or just a folder.
If you're more organized than I am, maybe you can do it systemwide. But I really like being able to turn it on for 30 days on my "Look at Later" folder, where I sort stuff that I might want to read.
I use RSS feeds wherever I can, rather than mailing lists. But my favored reader, Bloglines, doesn't have the ability to delete articles past X days. I have a few favorite feeds that I tend to read first since they only have a few listings, not dozens of them stacked up.
I may have to look at using Thunderbird for RSS, too. More on that later.
There are a few features of Outlook that I miss, mostly to do with calendars and tasks. I'm trying to get a unified calendar that works with Google (or something online) and also works well with my wife, who uses a Mac. So far, it's not been fun. More on that later, too.