A quick utility to map local email usernames to slack.com names

slackUserLookup.py

#!/usr/bin/env python
import sys
import simplejson as json
from urlgrabber import urlread

slackAPIToken = '<your API key here>'
emailDomain = '<your domain name here>'

try:
    lookupName = sys.argv[1]
except:
    print 'Usage: ' + sys.argv[0] + ': <user email to look up>'
    quit(1)

usersList = urlread('https://slack.com/api/users.list?token=' + slackAPIToken + '&pretty=1')

ul = json.loads(usersList)
for i in range(len(ul['members'])):
    if ul['members'][i]['profile']['email'] == lookupName + '@' + emailDomain:
        print ul['members'][i]['name']

You can use this with slacks incoming webhooks to target messages.

Incoming messages should have the channel set to '@' and the user will receive the message from slackbot.

If you plan on sending lots of messages like this, you'd probably be better off keeping a complete copy of the users.list locally and running off that, then update the local copy periodically. This approach would keep you well away from the API rate limit and unless users change their slack names frequently (who does that?) there should be few issues running that way.