Hi,Charles.
503 solved after moving again mailbox from one Exchange 2016 database to another
Do you mean that users also have the 503 error when open their mailboxes via Outlook? Please provide more detailed information, such as a screenshot if possible. (Notice: Please hide your personal information in the screenshot for security)
The problem may probably result from the Outlook Client continues to try to connect to the autodiscover service on Exchange 2010 server after migrated to Exchange 2016.
You need to change the DNS record of autodiscover service from Exchange 2010 to Exchange 2016.
And for domain-connected users, you also need to configure the scp record to point to Exchange 2016 server.
Below is the test in my lab.
It’s a co-existence environment of Exchange 2010 and Exchange 2016.
The “teste14” mailbox is created on Exchange 2010 and migrated to Exchange 2016.
When running a Test E-mail Configuration, we can see that Outlook first tries to connect to the autodiscover service on Exchange 2010 and failed.
Then I run the following command on Exchange 2010 Server to set the scp record to point to Exchange 2016 server.
Set-ClientAccessServer –Identity <Exchange 2010 Server name> -AutodiscoverServiceInternalUri https://<FQDN of Exchange 2016 Server>/autodiscover/autodiscover.xml
Run the test again and it connects to Exchange 2016 directly.
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