Mozilla Thunderbird is an excellent, open-source, and free mail client, which is available for many operating systems, including Windows, MacOS, and Linux.
If you're starting Thunderbird for the first time, it will ask you whether you want to import your mail and settings from another mail client, and will then prompt you to create a new account. Click cancel on both dialogs. Now Thunderbird will be loaded, and the setup will be the same whether this is an existing installation or new. Start by going to Tools-->Account Settings using the menus at the top of the Window. You should see the following window:
Click on Outgoing Server (SMTP) on the left, set the Server Name to smtp.yourdomain.com (where yourdomain.com is substituted by whatever domain you're setting up), type your E-Mail address as the username, and turn the Use secure connection option off (unless you don't mind certificate warnings):
Click on the Add Account button to bring up the following dialog. Ensure Email Account is selected and click Next >.
On the Identity page, fill in your full name and E-mail address. This is what will appear on your outgoing mail. Click Next > when finished.
On the Server Information page, change the server type to IMAP, and type in imap.yourdomain.com as the server name. Click Next > when finished.
Change the Incoming user name to your full E-Mail address, and click Next >
Simply click Next > on the Account Name page.
Your account is now set up and ready to go! The default settings in Thunderbird are sensible as they are, and your account is fully ready to use!
Old Mail Tip:
If you were using Thunderbird with POP3 previously, all of your old mail is still in Local Folders/Inbox. It is recommended that you move this into your IMAP Inbox to have all of your mail in one place, and so that it's stored in a safe place. To do this, simply go into Inbox under Local Folders, select all your mail, and drag it into the Inbox for the account you just configured. You should also move any old folders from Local Folders into your IMAP account.
