Steps to take prior to Installing Cumulative Update for Exchange 2010/2013/2016

I have come across the following issue post cumulative update for Exchange servers several times hence decided to share this with everyone.

Web.config is a file that contains client specific limits for OWA, EWS , and Active Sync. When a cumulative update is installed, the web config file is overwritten and the limits that you had in place prior to the upgrade needs to be reconfigured. So, it’s better to take a backup of the web.config file from the following locations prior to installing the cumulative update and also service packs as well to be on safe side.

After the update, you can rename the old web config to web.config.old or whatever you prefer and copy the web config file from the backup you took earlier. This must be done before you put the server back in production.

C:\Program Files\Microsoft\Exchange Server\V15\FrontEnd\HttpProxy\sync
C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Sync
C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\exchweb\ews
C:\Program Files\Microsoft\Exchange Server\V15\ClientAccess\Owa

For Exchange 2010, change V15 to V14.

https://docs.microsoft.com/en-us/exchange/architecture/client-access/client-message-size-limits?view=exchserver-2019

This process needs to be performed on Client Access servers on Exchange 2010/2013 servers but on Exchange 2016 it needs to be done on mailbox servers since both mailbox and client access servers roles are combined.

Testing

You should always test the following after the performing the above to make sure everything is good before placing the server back in Production.

1. Browse to https://localhost.owa/ and try to log in with a test account, it should work. Test mail flow as well.
2. Browse to https://localhost/microsoft-server-activesync to test activesync. You should get prompts for credentials and then you should get a http 501 error which is the correct response but if you get 403 forbidden something is wrong.