E-discovery search Exchange 2016 - too many mailboxes

You are getting that message when viewing the estimated search results ?

Any option to share the screen capture (hiding your personal information) to see when you are getting that message ?

We can retrieve the eDiscovery-related throttling policy parameters in your on-premises environment using the Get-ThrottlingPolicy cmdlet.

Get-ThrottlingPolicy | fl Compliance*, Discovery*

If the limitation it not so high, we can raise eDiscovery limits in Exchange Server as below steps:

In on-premises environments, you can modify throttling policies using the Set-ThrottlingPolicy cmdlet. But you can’t modify the global/default throttling policy. You’ll need to create a new throttling policy, set the intended parameters of the new policy and apply it to users who conduct eDiscovery searches. See Exchange workload management for more details about how throttling policies work in Exchange 2013.

Create a new throttling policy
This example creates a new throttling policy called test, sets the maximum number of mailboxes searched in a discovery search to 10,000 and maximum number of concurrent searches to 10.

New-ThrottlingPolicy test -DiscoveryMaxMailboxes 10000 -DiscoveryMaxConcurrency 10

Apply the new throttling policy
You still need to apply the throttling policy to users who’ll be performing eDiscovery searches. This example applies the new throttling policy to the user testuser1.

Set-Mailbox testuser1 -ThrottlignPolicy test

Members of the Discovery Management role group can perform eDiscovery searches. So it makes sense to apply the new throttling policy to all of them. This example applies the new throttling policy to all members of the Discovery Management role group.

Get-RoleGroupMember “Discovery Management” | Set-Mailbox -ThrottlignPolicy test

Verify the new throttling policy is applied to intended users
How do you know it worked? It’s easy to verify. This command lists all mailboxes that have the new throttling policy test applied. Because the Filter parameter doesn’t take the name of the throttling policy (test, unlike the Set-Mailbox cmdlet), we capture its DistinguishedName property in the $policy variable and use the variable in the recipient filter.

$policy=(Get-ThrottlingPolicy test).distinguishedName

Get-Mailbox -ResultSize unlimited -Filter {ThrottlingPolicy -like $policy} | ft name,ThrottlignPolicy -Autosize

Hope it helps.

but the default policy is 5000. We only have 1500 mailboxes. see attached error.
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Create a new policy and check by setting the mailbox to the new policy as decribed below. Don't work with the default policy for now.

Create a new throttling policy
This example creates a new throttling policy called test, sets the maximum number of mailboxes searched in a discovery search to 10,000 and maximum number of concurrent searches to 10.

New-ThrottlingPolicy test -DiscoveryMaxMailboxes 10000 -DiscoveryMaxConcurrency 10

Apply the new throttling policy
You still need to apply the throttling policy to users who’ll be performing eDiscovery searches. This example applies the new throttling policy to the user testuser1.

Set-Mailbox testuser1 -ThrottlignPolicy test

Members of the Discovery Management role group can perform eDiscovery searches. So it makes sense to apply the new throttling policy to all of them. This example applies the new throttling policy to all members of the Discovery Management role group.

Get-RoleGroupMember “Discovery Management” | Set-Mailbox -ThrottlignPolicy test

https://social.technet.microsoft.com/Forums/en-US/8e76bce4-da61-4fff-9c18-bd49199fafb3/unable-to-perform-discovery-search-on-all-mailboxes?forum=Exch2016Comp

Instead of All Mailbox on the search.. Select the distribution Group that has all the members in your in Exchange..