Popup in the regular XMPP clients each time somebody tries to start a new chat with a user @gmail.com and doesn't receive a response in 2 minutes. "Warning: the person you have tried to contact uses gmail.com. It is possible that their account has been migrated to the Hangouts platform and that Google has silently discarded chats you send them. …

mod_hangouts.lua

-- by Kim Alvefur
-- Popup in the regular XMPP clients each time somebody tries to start a new chat with a user @gmail.com and doesn't receive a response in 2 minutes.  "Warning: the person you have tried to contact uses gmail.com.  It is possible that their account has been migrated to the Hangouts platform and that Google has silently discarded chats you send them. Please click here for a help page you can email to your contact to fix their account."
-- This logic can be refined. The broken resources start with "Messaging".

local st = require"util.stanza"
local jid_split = require"util.jid".split;

module:hook("pre-message/full", function (event)
        local origin, stanza = event.origin, event.stanza;

        local node, host, resource = jid_split(stanza.attr.to);
        if resource and resource:match"^Messaging" then
                return origin.send(st.error_reply(stanza, "cancel", "not-acceptable",
"One does not simply chat with a Hangouts user"));
        end
end);

local function drop_hangouts_presence(event)
        local node, host, resource = jid_split(event.stanza.attr.from);
        if resource and resource:match"^Messaging" then
                return true -- Drop
        end
end

module:hook("presence/bare", drop_hangouts_presence);
module:hook("presence/full", drop_hangouts_presence);