You already have Outlook Express on your machine. It's built-in to XP. It's very basic and reliable as long as you keep it neat & clean.
General precautions for Outlook Express:
Do not archive mail in the Inbox or Sent Items. Create your own user defined folders and move the messages you wish to save to them. Empty Deleted Items folder daily. Although dbx files have a theoretical capacity of 2GB, I recommend about a 300MB max for less chance of corruption.
Information about the maximum file size of the .dbx files that are used by Outlook Express:
http://support.microsoft.com/?kbid=903095
Compact often.
Click on Outlook Express at the top of the folder tree so no folders are open. Then: File | Work Offline (or double click Working Online in the Status Bar). File | Folder | Compact all folders. Don't touch anything until the compacting is completed.
Turn off e-mail scanning in your anti-virus program. It is a redundant layer of protection that eats up CPUs and causes a multitude of problems such as time-outs and account setting changes. Your up-to-date A/V program will continue to protect you sufficiently. For more, see:
http://www.oehelp.com/OETips.aspx#3
And backup often.
Outlook Express Quick Backup (OEQB Freeware)