Mailpile
Mailpile is a modern, fast web-mail client with user-friendly encryption and privacy features.
Project Source: https://github.com/pagekite/Mailpile
Project Homepage: https://www.mailpile.is/

kite
Kite is a webmail designed to look a lot like gmail and to be easily deployable on a single server.
Project Source: https://github.com/khamidou/kite
Project Homepage: http://kiteapp.io/

inbox.py
Python SMTP Server for Humans.
Project Source: https://github.com/kennethreitz/inbox.py
Project Documentation: https://pypi.python.org/pypi/inbox/

gmail
A Pythonic interface to Google's GMail, with all the tools you'll need.
Project Source: https://github.com/charlierguo/gmail

gmvault
Gmvault is a tool for backing up your gmail account and never lose email correspondence.
Project Source: https://github.com/gaubert/gmvault
Project Website: http://gmvault.org/

imbox
Python library for reading IMAP mailboxes and converting email content to machine readable data.
Project Source: https://github.com/martinrusev/imbox.

mailin
Artisanal inbound emails for every web app.
Project Source: https://github.com/Flolagale/mailin
Project Homepage: http://mailin.io/

envelopes
Mailing for human beings.
Project Source: https://github.com/tomekwojcik/envelopes
Project Documentation: http://tomekwojcik.github.io/envelopes/

mailur
Webmail client with Gmail-like conversations.
Project Source: https://github.com/naspeh/mailur
Project Homepage: http://pusto.org/en/mailr/

lamson
Pythonic SMTP Application Server.
Project Source: https://github.com/zedshaw/lamson
Project Homepage: http://lamsonproject.org/