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
TonyRedmond Sorry that might have been a confusing oversimplification on my part. They weren't disconnections as such. I've just checked our logs and we were randomly seeing the error
The SMTP server requires a secure connection or the client was not authenticated to send mail. Error 535 5.7.139 Authentication unsuccessful...
This was using an account that had previously authenticated and was able to again later on with absolutely no changes on our part.
Oh and also as per my comment above there is apparently no way to retrieve NDRs
Unfortunately we don't have the resources for more testing and I can't risk using this in Production. I will wait until HVE hopefully reaches general release.