Ron is having issues with Thunderbird after a recent security change that is causing him issues. Leo says that Thunderbird has largely been abandoned by developers, who are simply not keeping it up. He may want to check his IMAP and SMTP addresses to make sure they are configured correctly. He should double check how to properly log into it, and what port he'll need to use. His ISP can help him with that.
Other email choices include Pegasus, Claws, and Microsoft Outlook Mail. The Bat! by RITLabs.com is another option. They have a try before you buy feature. $26.95 for a Home license and $59.99 for the Professional.