Hi Alex,
Please run Office 365 Outlook Autodiscover test. If you get any error message, please let me know.
Moreover, I'd like to confirm the following things:
1. How long have you registered for this trial version of Office 365?
2. Can you send and receive emails from Office 365 OWA?
3. Which version of Outlook do you use, Outlook 2010 or Outlook 2013?
Note: if you have just registered for the Office 365 trial, the new created DNS records may take up to 72 hours to take effect, so wait for more time, and then test it again.
Thanks,
Edward Qu