For latest updates on HVE, please see this post as well as this post.
Today we’re thrilled to announce the public preview of High Volume Email (HVE) for Microsoft 365. HVE is a new service designed...
Updated May 06, 2025
Version 5.0
Blog Post
Peter_Holdridge thank you for your comment, especially that security is our top priority. With the HVE we are separating HVE traffic from your Tenant mailboxes. With HVE we are providing dedicated end point which do not interact with mailboxes. Based on the documentation:
If Security Defaults is enabled, all basic authentication including SMTP is disabled, making High Volume Email (HVE) non-functional.
HVE accounts can operate even if SMTPClientAuthenticationDisabled is set to True within TransportConfig, due to the use of the custom endpoint.
To avoid any impact from authentication policies, it's important for customers to ensure AllowBasicAuthSmtp is enabled in their organization for the HVE account. Custom authentication policies can be applied to the HVE accounts as needed.
If there is a need to be fully (physically) separate from your tenant, you can use Azure Communication Services Email with also dedicated SMTP end point.