There is Geary, but I have tried it and it is both lacking in features and pretty buggy. It is quite nice looking however.

Myself I have all but given up on GUI email clients, I still use Thunderbird from time to time but only when I need PGP encrypted email, otherwise I stick to webmail. I am looking forward to Mailpile if it delivers on its goals, looks very promising.

http://www.indiegogo.com/projects/mailpile-taking-e-mail-bac...