Does Zimbra desktop and Yahoo Mail Plus work seamlessly with Comodo Securemail.
As per Comodo website
It is a desktop application that automatically secures your email messages so your confidential data cannot be stolen or modified on its way through the Internet.
Security you install and forget - SecureEmail will automatically encrypt and sign all outgoing emails and manage the complex certificate exchange procedure
Full integration with major mail clients - Outlook, Outlook Express, Thunderbird, Incredimail and many other POP, SMTP and IMAP based clients
Encrypt to anyone, anytime - innovative one-time 'session' certificates allow you to encrypt email without first exchanging signed mails
Built-in wizard to lets you quickly download and install Comodo email certificates