Recently a user needed to Search another user’s 100GB Archive and Mailbox. Outlook search was taking far too long.

You can assign a user Permission to use the Content Search in  Security and Compliance Center  and limit access to a single Mailbox.

Assign Roles : 

In the Security and Compliance Center, Under Permissions,  Create a new Role with the following Roles assigned and add the User who needs to access

Assign Filter

This limits the users search scope to only a single Mailbox

Connect to 365 Security and Compliance Center :

$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.compliance.protection.outlook.com/powershell-liveid -Credential $UserCredential -Authentication Basic -AllowRedirection Import-PSSession $Session -AllowClobber -DisableNameChecking

Add Filter

New-ComplianceSecurityFilter -FilterName MailboxFilter -Users [email protected] -Filters “Mailbox_DisplayName -like ‘John Smith'” -Action All

Wait around 10 minutes for this to apply then try logging in