https://docs.microsoft.com/en-us/exchange/security-and-compliance/messaging-records-management/mailbox-retention-hold

Check for holds:

Get-Mailbox -ResultSize unlimited | Where-Object {$_.RetentionHoldEnabled -eq $true} | Format-Table Name,RetentionPolicy,RetentionHoldEnabled –Auto

Disable:

Set-Mailbox user -RetentionHoldEnabled $false