Trying to compile a list of  secure email clients based on feedback from users.

Most corporations use :
Groupwise
Lotus Notes
Microsoft Exchange

For home users and small businesses, the popular email clients are:
Outlook Express -  Not suitable for a large number of emails.
Thunderbird – common error, This folder is being processed. Please wait till processing is complete to get messages.
MS Outlook -  seems to be the least secure email client, can easily be hacked.

Plain text is always the best, the possibility of malicious scripts getting downloaded are minimized. Other options are:
Poppeeper.com
PocoMail
Popcorn by Ultrafunk
Kmail (for KDE on Linux)
Pine (replaced by Alpine)