I have a domain that I was using google apps for but decided i'd rather use a paid provider for the email as I wanted a support department available just in case something goes wrong or I run into a problem.
I switched it over last week, changed the MX records and then disabled Email from the domain control panel.
if you try and access the google apps interface it rightly tells you that email is not enabled for this domain... BUT Today I turned my laptop on for a first time in a while (so it still had the old settings in thunderbird) i was able to successfully authenticate against smtp.gmail.com using
myname@domain.com and send an email and it was revived ok by the recpient
Kind of worrying as that account is supposed to be disabled for email, and yet the login is still usable on the SMTP
I think IMAP did also auth ok, although it didn't appear to sync the folders with what was on the server (otherwise a load of stuff would have been deleted from the laptop)