Multiple Email Accounts - How Do I Do It?

I'm trying to work out my email configuration. As long time readers know, I have an old PC repurposed as an IMAP server. I have four logins on the server, and each is used to catch email via Fetchmail. On the desktop machine, I use Mozilla Mail with an acount setup for each (Personal/family email, junk address, e-commerce address, school address). A fifth account in the client hooks to my work mail server. I also get to my email via my webmail setup (SquirrelMail - It's Webmail for Nuts!) that's hosted on my work web account.

I do this not because I need to keep the messages separate, but because I want to be able to choose which is used as the FROM: address when sending mail. I'd like replies to go to whatever address the message came TO:, and I should be able to choose the FROM: address on a new message.

The personal is just used for family and close friends (no spam here). The e-commerce address is used when buying things online and registering at trusted sites (like newspapers). The junk address can be used anywhere where I think the address may be sold or grabbed for spam use. The school address keeps me from having to give my students the personal address - just in case one gets pissed at me and sends it out.

Do I have to do it this way? Is there a way to be able to send out from any of several addresses while the mail sits in just one mail account on my server? I would have to be able to see the FROM: address on each incoming message - similar to the way Outlook can be configured to show any of a long list of columns.

Let me know how you have handled this issue.