Someone posted the source code for a reddit bot a few days ago that can do the job. I can't find it right now, I'll have to look around.

Edit: I think it was this

import praw
import requests
''' Variables '''
PM_SUBJECT = 'SUBJECT'
PM_MESSAGE = """ MESSAGE """
USERAGENT = 'My Reddit Bot 942'
''' Code '''
print('Logging in...')
r = praw.Reddit(client_id='YOU MUST GENERATE THIS', client_secret='SAME HERE',user_agent='automates account function',username='YOURNAME',password='YOURPASSWORD')
print(r.user.me())
f = open("users.txt", "r")
f1 = f.readlines
for x in f1:
print(x)
try:
    r.redditor(x).message(PM_SUBJECT,PM_MESSAGE)
except:
    print("Account Deleted?")
print("MESSAGE SENT")