"#2: Claws Mail
Claws Mail is a free, GTK+-based, open source email and news
client. It is very light lightweight. Like Firefox , the wide
variety of plug-ins for this email client makes it very flexible
and secure. Claws Mail runs on Windows, Mac OS X and Unix-like
systems such as Linux, BSD, and Solaris.
Spicebird is a collaboration client that provides integrated access
to email, contacts, calendaring and instant messaging in a single
application. It provides easy access to various web services while
retaining all the advantages of a desktop application. It is
developed by an Indian company called Synovel. It is a free, open
source and cross-platform software."