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Microsoft 365 Blog
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High Volume Email Is Now Available in Exchange Online

JeremyCarlson's avatar
JeremyCarlson
Icon for Microsoft rankMicrosoft
Mar 31, 2026
Reliable email remains one of the most critical ways organizations communicate with employees at scale especially for operational, time‑sensitive messages like payroll notifications, security advisor...
Updated Mar 31, 2026
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Larry Heier's avatar
Larry Heier
Copper Contributor
Apr 13, 2026

We are switching to HPV but have a suggestion/question.

Has there been thought to create an SMTP relay service/application for on-premise to replace Exchange SMTP.?   I have environments where port 25/587 is locked down save emailing to M365 and having one or two (HA) trusted SMTP applications would help with a smoother migration to the new HPV service.  I know of one of my customers that has well over 100 SMTP Relays to now update.   It is a good amount of work to switch over every SMTP relay to HPV or Azure and some of the older scanners/applications may not support authenticated SMTP relay.

thanks in advance,

Larry Heier

JeremyCarlson's avatar
JeremyCarlson
Icon for Microsoft rankMicrosoft
Apr 17, 2026

Thanks for raising this — it’s a very real migration challenge, especially in environments with a long tail of on‑premises devices and applications.

High Volume Email (HVE) was designed to solve high‑scale, authenticated, application‑generated email for Microsoft 365, but it is not intended to be a drop‑in replacement for Exchange’s on‑premises SMTP relay role. In particular:

  • HVE requires authentication (OAuth or account credentials)
  • It does not support anonymous relay
  • It is scoped to internal recipients only
  • Older devices that only support unauthenticated SMTP are out of scope by design

For customers with on‑premises environments that still rely on:

  • Anonymous or IP‑based relay
  • Large numbers of existing SMTP relay endpoints

The supported migration paths today remain:

  • SMTP relay using Microsoft 365 (via tenant MX with IP allow‑listing), or
  • An intermediary relay layer (for example, a small number of highly‑available SMTP relay servers or services) that legacy devices point to, which then handles authenticated delivery to Microsoft 365 or HVE.

The feedback around easing migration — especially for customers with many legacy relays or devices that can’t support authenticated SMTP — is absolutely valid and has been heard.