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
We started having delivery failures to gmail when we switched to HVE. It turns out the HVE service adds its own 'Date' header, even though we'd already added one. Our 'Date' header is supposed to mark the time the message was created in our system, rather than the time the message was finally submitted to Microsoft. The duplicate 'Date' headers results in this error:
> 550 5.7.1 This message is not RFC 5322 compliant. There are multiple Date headers. To reduce the amount of spam sent to Gmail, this message has been blocked.
"Not RFC 5322 compliant" - I'd call that a bug. Hopefully it will be fixed.
Simone_LiebalMicrosoft
rld we don't add headers within the HVE service. Could you please open a support ticket on this, so we can investigate this further?