Did you ever ask yourself how Outlook connects to your servers

Outlook uses AutoDiscover service to connect to Exchange online or you on premise Exchange

In some scenarios, you may want to control the methods that are used by Outlook to find the AutoDiscover service. This depends on the client/server topology, but these are the methods that are used by Outlook:

By default, Outlook uses one or more of these methods to reach the AutoDiscover service. For example, for a computer that is not joined to a domain, Outlook tries to connect to the predefined URLs (for example, https://autodiscover.contoso.com/autodiscover/autodiscover.xml) by using DNS. If that fails, Outlook tries the HTTP redirect method. If that does not work, Outlook tries to use the SRV record lookup method. If all lookup methods fail, Outlook cannot obtain “Outlook Anywhere” configuration and URL settings.

Lets understand the 5 methods that outlook uses:

SCP lookup
Outlook performs an Active Directory query for Service Connection Point (SCP) objects.

Root domain query 
Outlook uses the root domain of your primary SMTP address to try to locate the AutoDiscover service. Outlook tries to connect to the following URL based on your SMTP address: https://<smtp-address-domain>/autodiscover/autodiscover.xml

AutoDiscover domain Query
Outlook uses the AutoDiscover domain to try to locate the AutoDiscover service. Outlook tries to connect to the following URL based on your SMTP address: https://autodiscover.<smtp-address-domain>/autodiscover/autodiscover.xml
HTTP redirect
Outlook uses HTTP redirection if Outlook cannot reach the AutoDiscover service through either of the secure HTTPS URLS:
https://<smtp-address-domain>/autodiscover/autodiscover.xml
https://autodiscover.<smtp-address-domain>/autodiscover/autodiscover.xml

SRV record query
Outlook uses an SRV record lookup in DNS to try to locate the AutoDiscover service.

Now how can you control these methods

You can do this and enable/disable these methods for outlook through registry keys

Outlook 2010 :
HKEY_CURRENT_USER\Software\Microsoft\Office\14.0\Outlook\AutoDiscover

Outlook 2013 :
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\AutoDiscover

Outlook 2016:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\AutoDiscover

You can change the various options from the registry file as well. They are:

“ExcludeScpLookup”=dword:00000001
“ExcludeHttpsRootDomain”=dword:00000000
“ExcludeHttpsAutoDiscoverDomain”=dword:00000000
“ExcludeHttpRedirect”=dword:00000000
“ExcludeSrvRecord”=dword:00000000

Above the SCP lookup is disabled.

Source : Support Article , DISABLE AUTODISCOVER METHODS IN OUTLOOK 2013