In case you run into slow account setup with Outlook 2016, which can take up to 10 mins or more. To workaround this issue, you may use the following Registry settings to force Outlook to bypass the root domain discovery and redirect to the autodiscover CNAME or A record to resolve the address for autodiscover settings.

For Outlook 2016:

Note: For Office PC that has joined the campus Microsoft Active Directory, there is no need to amend the registry. This setting will be applied via the group policy when you login the system.

It is assumed this is caused by some bug in Outlook 2016 that causes the system to try a little too hard to obtain a response from the Autodiscover directory on root domain. If the domain resolves in DNS but does not respond to HTTP queries (i.e. you don't have an actual server to acknowledge them), this will cause timeout error and Outlook will try repeatedly.

If you wait long enough (11 attempts or so, each taking at least 30 seconds), Outlook will actually give up and move on. But if you don't like waiting, the reg key above will force Outlook to bypass this particular check and it will continue to the next method. So as a result the process will take a substantially smaller amount of time.

For more information, you can refer to: http://www.mistercloudtech.com/2015/12/08/how-to-resolve-slow-office-2016-autodiscover-with-office-365