This widget shows count of new mail in your mailbox, checked via python script by IMAP

Python script

#!/usr/bin/python

import imaplib

#first field is imap server, second - port (993 for gmail SSL IMAP)
M=imaplib.IMAP4_SSL("imap.gmail.com", 993)
#first field is imap login (gmail uses login with domain and '@' character), second - password
M.login("user@gmail.com","password")

status, counts = M.status("Inbox","(MESSAGES UNSEEN)")

unread = counts[0].split()[4][:-1]
if int(unread) == 0:
    print "  0  "
else:
    print "<span color='red'>  <b>"+unread+"</b>  </span>"

M.logout()

Save this script in ~/scripts/unread.py and set executable permission to it.

rc.lua configuration

--- Mail updater
mymail = widget({ type = "textbox", align = "right" })
mymail.text = "  ?  "

Hook (check mail every 30 seconds)

awful.hooks.timer.register(30, function ()
    local f = io.open("/home/YOUR_USER/tmp/gmail") 
    local l = nil
    if f ~= nil then
       l = f:read() -- read output of command
    else
       l = "  ?  "
    end
    f:close()

    mymail.text = l
    os.execute("~/scripts/unread.py > ~/tmp/gmail &")
end)