Balsa
Claws Mail
Birdtray
GNOME GMail
Phone Manager
Jami
Gnubiff
Dino
HexChat
qTox
KVIrc
Linphone
Loqui
Mumble
Thunderbird
Notmuch (emacs interface)
Gajim
Evolution
Geary
Polari
KMail
Konversation
KTnef
PIM Data Exporter