We discovered a problem while creating a new user in our on-prem Exchange 2013 ECP. I amused myself in resolving the issue and have decided to inflict it upon the world.
On trying to create a user, the following error appeared.
I had recently been working with the related databases, so this piqued my curiosity. While I searched for answers, a co-worker discovered that you could manually select a database from the "More options" text, just below the *User logon name" field. This did indeed work for bypassing the problem. I didn't want to leave it like this, as I know Exchange uses each database in turn to distribute the load. It probabaly doesn't matter in my environment, but it's not right, and I needed to fix it.
Some quick research led me to a technet thread discussing something related, but not useful directly.
Near the end of the thread, I learned of a flag on the databases I'd never noticed before.
Set-MailboxDatabase "MailboxDatabaseName" -IsSuspendedFromProvisioning $false
Set-MailboxDatabase "MailboxDatabaseName" -IsExcludedFromProvisioning $false
Checking my databases, I discovered that indeed they were excluded from provisioning.
Get-MailboxDatabase | FT Name, IsExcludedfromProvisioning, IsSuspendedFromProvisioning
On discovering this, I did some additional research to find out when those flags are used. Not finding anything other than user creation, I proceeded to modify the excluded flag on both databases like so.
get-mailboxdatabase | Set-MailboxDatabase -IsExcludedFromProvisioning $False
Running the get-mailboxdatabase command again with FT Name, IsExcludedFromProvisioning resuled in getting the expected false result. Testing user creation also passed with flying colors.
I fixed a thing! I wish I understood how any of my previous changes set that flag as true, but alas I have no answer at this time. But I learned a great deal of other useful information!