"The mailbox has additional shards that need recovery" error when recovering an inactive mailbox

Symptoms

When you try to recover an inactive mailbox in Exchange Online, you receive the following error message about multiple component shards:

The inactive mailbox '<identity of the inactive mailbox>' cannot be recovered because the mailbox has additional shards that need recovery.

Cause

This issue occurs if the inactive mailboxes have AuxPrimary shards in addition to Primary shards.

To see which inactive mailboxes are affected by multiple shards, you can check the mailbox location information by running the following cmdlet in Exchange Online PowerShell:

Get-Mailbox -InactiveMailboxOnly | fl UserPrincipalName,MailboxLocations

In the following example output, the mailbox shard location type is shown as AuxPrimary and Primary. This means that the mailbox has AuxPrimary shards.

Resolution

To resolve this issue, restore the inactive mailbox instead of recovering it.