Sending DMs from an Application



I’m developing an Application, with some parts based on Twitter.
First part : My users can authorize me (Read/Write permission). They can set up a few filters (ie: blacklist hashtags, and so on) and read their filtered timeline.
Other part : My users can register their websites, my app checks (ie every 1 hour) if the site is down or online,
and I want to warn them by email/twitter/sms that their site is currently down.
(Some users asked for an alert via DM on Twitter - mentions are NOT ok, as they don’t want URLs to appear in a public space)

I created a Twitter Account for my app. Is there a way to be able to send DMs (my users already follow the “official account”) without moving the WHOLE app to a Read/Write/Access DM mode ? Do I need to create a second App and authorize only my official account ?

Right now, when I try to send a DM, I get a “Unable to authenticate (32)” error. I can tell that the tokens are fine because /account/verify_credentials.json works.

Thank you very much.