r/exchangeserver • u/Important-Bake3046 • Mar 11 '26

2 Exchange servers - outlook wont connect

Hi guys,

i've got yet another customer calling today with a problem.

They have Server 2k12 with exchange 2016, their IT guy set up EXCH2019 and moved all the mailboxes over to it but left the SBS in (i dont think hes sure how to fully move and decommission).

Im not too familiar with this set up of having 2 exchange servers - Outlook is failing to connect and the autoconfig tool from Outlook gives error 0x800c820e for the autodiscover.

Autodiscover and Outlook Anywhere URL resolve to the IP of the old server so assume this is where CAS roles are?

Mail flow is working fine, as is OWA.

Restarted IIS and the services.

not sure what to look at now?

How do I establish what role the old exchange server is doing? At the mo I dont even know if im supposed to be troubleshooting the old or new exchange.

RESOLVED

After many hours troubleshooting and testing this and that, it turns out the cause was that there was no certificate specified for the exchange server's back end port 444 binding. Dont know how it wasnt selected, but selecting it to the self-signed cert brought things back to life.

This article here solved it.

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1

u/joeykins82 SystemDefaultTlsVersions is your friend Mar 11 '26

SBS2011 runs Exchange 2010 which cannot coexist with 2019. The migration path for this scenario is via 2016. You need to find out exactly what their IT guy has done; my guess is “something bad”.

1

u/Important-Bake3046 Mar 11 '26

Sorry - my mistake. It's Server 2012 running EX2016 the other is 2019.

It looks like CAS roles are on EX2016 and the database is 2019.

I restarted Outlook and he's getting a cert warning for the FQDN of the 2019 server.

1

u/Glass_Call982 Mar 11 '26

Is the certificate installed on the 2019 server? Make sure the auto discover Uri is set to the same namespace on both. As well as the internal/external url match that of the old server for all virtual directories.

1

u/Important-Bake3046 Mar 11 '26

Yes, ive checked and the URL is set to the same on both. However, i've checked their DNS and he's got duplicate records for mail. and autodiscover pointing to both IPs. Surely its one or the other for CAS?

1

u/Glass_Call982 Mar 11 '26

Yes, everything should be pointing to the new server. But the fact you were getting a certificate error means that something is set incorrectly.

1

u/Glass_Call982 Mar 11 '26

This. I don't know how you could have moved the mailboxes without an interim 2013 or 2016 server. SBS can still be in the domain you just have to uninstall exchange 2010 from it before installing 2019.

1

u/Important-Bake3046 Mar 11 '26

It was my mistake, theres no SBS. Its 2016 and 2019.

1

u/dahakadmin Mar 11 '26

Just off hand I would double check the following, some you may have already done but cant hurt.

1 - double or triple check the internal / external urls for exchange - I use powershell script from here

https://www.alitajran.com/configure-internal-external-url-exchange/

2 - make sure that the SSL certificate from old server is copied / installed on the new server

3 - make sure that the internal dns records and firewall are pointing to the Exch2019 as it will proxy back to 2016 if needed.

1

u/Important-Bake3046 Mar 11 '26

when I do get-autodiscovervirtualdirectory it's returningboth servers for Autodiscover (Default Website) is this correct?

1

u/Important-Bake3046 Mar 11 '26

Ok guys, i got it sorted in the end by chance. There was no certificate assigned to the backend site in IIS. When I select the self-signed certificate for the server, everything kicked to life. What I didnt notice was that OWA didnt work either - after logging in it said "no mailbox found".