There have been a few times that I couldn't retrieve my e-mail through Outlook Express even though the mail server was running just fine. I wasn't experiencing DNS problems and every other program I had could access the internet just fine... After a bit of frustration I gave up but my friend Marc was able to figure it out!
If you're experiencing problems with your mail client connecting to your mail server (OE says cannot find host) and you know it's up, there's a simple fix. First you have to locate your 'hosts' file (searching for it via Windows search tool is easiest), please keep in mind that the file has no extension and in WindowsXP it's in your Windows\system32\drivers\etc directory.
Once you find it open it up with notepad and scroll down to the bottom. Enter the ip address of your mail server then press the tab button then enter the name of your mail server (for instance my ISP mail server is stmp.rogers.com). Once that's done save and exit and your mail client should work now.
If you don't know the IP address of your mail server you'll have to get a friend to do an nslookup (start -> run, CMD and press ok). From the command prompt type "nslookup stmp.rogers.com" but instead put your mail server's address.