When a labeled document is uploaded, copied or moved to a SharePoint library classified with a default label with higher priority than the document itself, these emails are sent to the document owner. Microsoft's intention to notify the owner about an incompatible label applied makes sense to me. But it cannot be applied to every scenario.
A typical workflow of our customers is to prepare and create documents in a sort of internal SharePoint site and then copying or moving them to a workspace, where documents are for example shared with B2B partners or other external parties for collaboration. If the document was labeled by a default labeling policy in the "internal" workspace and then is copied to an "external" workspace with a lower prioritized default label applied, these messages start to bug users, and you will get more support calls, which you want to avoid. I think it's valid, that you would like to keep a file better protected that the lower standard on the container. I would consider the above described scenario as a good practice, and Microsoft provides us with an option to disable these email notifications if not required. Be aware that this will also disable the audit log event captured for now.
Use the following PowerShell commands to enable or disable these standard emails on a tenant level. This is a SharePoint setting, so it will apply only there and in OneDrive. But when writing this article, this was the only place which triggered such an email anyway.
Prerequisites:
Commands:
1#Connect to SharePoint Tenant Admin
2Connect-SPOService -Url https://SPBASEURL-admin.sharepoint.com
3
4#Get the current status of the setting
5Get-SPOTenant | Select-Object BlockSendLabelMismatchEmail
6
7#Block the standard email
8Set-SPOTenant -BlockSendLabelMismatchEmail $true
9
10#Enable the standard email
11Set-SPOTenant -BlockSendLabelMismatchEmail $false