I have a set of accounts with a custom domain name. ONE of them can't log in from an app (phone mail app, mac mail app) only the browser. What could be wrong? 

I did have a whole range of DNS problems with my provider overwriting my mx records. I fixed the mx records, mail is being delivered, but I still can't log in from my mail app.

Any ideas? Do I need a CNAME record or something to direct my app to the right place??