After a failover or similar event, you may find that a passive mailbox database copy has a ContentIndexState of Failed or FailedAndSuspended in your Exchange 2016, 2013 or 2010 Database Availability Group.
In our case, we have four mailbox database copies and one of them, MDB03, has a ContentIndexState of FailedAndSuspended:
Get-MailboxDatabaseCopyStatus | sort Name | ft -AutoSize
To resolve this, we need to use the Update-MailboxDatabaseCopy cmdlet and specify the -CatalogOnly switch so that only the content index is re-seeded rather than the entire database:
Update-MailboxDatabaseCopy MDB03LITEX01 -CatalogOnly
After this completes, check your content index state:
Get-MailboxDatabaseCopyStatus | sort Name | ft -AutoSize
As we can see, all content indexes are healthy and our issue is resolved.