Which are the most popular email programs?

Mate, email programs are really cool it is used evolution because it is very well integrated. For example, the calendar that drops down when you click on the clock on the right of the top GNOME panel bolds days that have appointments, and lists them if the day is selected. It also shows pending to-do items.

So, if you look in synaptic, gaim has an evolution plug-in so you can synchronize your buddy list with the address book in evolution. Secondly there is a KMail it is also very good, I used it for years. It's very well integrated with the rest of KDE, and if you want a full PIM application with calendar/tasks/address book etc. you can run Kontact. Thunderbird is really cool because of all the extensions that are available for it. Thunderbird's EnigmailPGP/GnuPG extension offers far more flexibility to Evolution's PGP support. And lastly there are also Balsa and Sylpheed, but to be frank I haven't tried them out.