When is an AUTO-DISCOVER record necessary?

Hello - I am reading that an auto-discover record is required now for Outlook 2016/19 to connect to your Exchange Server.  I need to understand if this pertains to BOTH Exchange Online AND Exchange Server (on prem)? or are they just meaning for Exchange ONLINE only, an autodiscover record is needed in DNS?  The specify the format of the record as "autodiscover.domain.com", which would only apply to Internet searches, and not within our internal network, correct?  Looking for some clarification on this, as to what do I need for my clients on my network internally to find the Excahnge Server locally.  

Thank you,
Damian GardnerIT AdminAsked:
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Adam BrownSenior Systems AdminCommented:
Autodiscover is required after Outlook 2016 for *all* Exchange deployments, whether on-prem or in cloud. I've written a lot of posts about the autodiscover service and how to work with it on my blog: https://acbrownit.com/?s=autodiscover will show all of them, and you can go through at your leisure.
Damian GardnerIT AdminAuthor Commented:
ok good to know.  readin them now.  thanks Adam
Jose Gabriel Ortega CastroEE Rookie/Solution Guide/Topic Advisor and CEO Faru Bonon ITCommented:
HI adam

Here's the fact.

To understand autodiscover there's a simple fact
Autodiscover is required to ActiveSync (phones), and outlook connectivity

So autodiscover is a process that goes into 4 steps:
  1. domain.com (it will look for https://domain.com/autodiscover/autodiscover.xml" if it's found it will "autoconfigure your clients" if it's not found goes to step2
  2. Step2 is similar to 1, just that it will search on https://autodiscover.domain.com/autodiscover/autodiscover.xml", if it's not found it will do step3
  3. Step3 is all based on Http redirection (this one is particularly useful because it will query locally and if it's not found it will redirect to Office 365 (Exchange online) and if it's not found...
  4. Step4 This is based on DNS SRV redirection, particularly useful when you have 2 domains (domA.com and domB.com), but you are poor and you just got a wildcard certificate for domA.com, (so your domB.com is in the air), then you go to the domB.com public DNS and add an SVR Record that goes to the domA, like mail.domA.com, and solves the situation for domB.)

After all of this as you can see, each component of the autodiscover process attacks a different problematic, steps 1 and 2 are for locally step3 is for O365 and step4 is for poor hahaha :P (well it's an expression, I personally use 4 a lot :) especially with customers that doesn't see the added value when you do the different SANs in the same SSL).

So When do you need an autodiscover record, will always depend on the customer requirements. why? because if they want an exchange server with a single 5 domains but they only want Outlook web access, then you won't do any record because connectivity with outlook is not in the customer requirements. ( the number of domains only affects the autodiscover record, and you configure it internally and externally using the main domain of the SSL, it will get the rest automatically).

Now if the client wants a multi-domain configuration and he wants to buy a single SSL for 5 domains then it will need to support 10 Subject Alternative Names (SANs). which ones?, well:
(repeat for all 5)

So the answer is just, it will depend if the customer wants to be able to connect their outlooks or if they preferred to work just with the office network..

All 4 steps can be seen here:
Test "Exchange Server/Outlook Autodiscover"

I made up a test

4 errors mentioned on the Autodiscover process

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Damian GardnerIT AdminAuthor Commented:
Thanks guys for your help on this.  Much appreciated!
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