Error sending email: SMTP Error: Data not accepted.

I'm not sure if this issue is for all setup's, but we also use the SMTP Auth module and when attempting to send broadcast emails with the advuser module, we were getting the following `notice` in our admins log - even though the form submitted successfully:
Error sending email: SMTP Error: Data not accepted. From: To: bill@example.com

So I had a look at the code, and in the user_mail call within the advuser_multiple_email_confirm_submit function (around line 332), there was a reference to a '$from' variable, which is never set. So I just added the following code before the user_mail call, and it seems to have fixed it:
$from = variable_get("site_mail", ini_get("sendmail_from"));

Hope it can be of some use to someone.