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High-Volume Email for Internal Recipients Only – And It Costs
Almost two years ago, Microsoft launched the High-Volume Email (HVE) solution for Exchange Online. The idea was very simple. Exchange Online is not designed to host mailboxes that send very large amounts of email. Personal (and shared) mailboxes don’t generate that kind of traffic – or at least, they shouldn’t in a shared resource environment like Microsoft 365. This is why traffic thresholds like the tenant external rate limit exist (Microsoft proposed to implement a mailbox-specific external rate limit but cancelled the plan in January 2026).
However, customers do have requirements to send large amounts of email from time to time, and that’s what HVE is all about: a separate, paid-for service built on top of Exchange Online that can handle hundreds of thousands of emails daily. HVE can handle anything from internal employee communications to sending status updates from applications. The message in Figure 1 was generated using this script.
When it first went into preview, the High-Volume Email solution had a limited ability to send email to external recipients. Microsoft removed that capability in 2025 to simplify HVE and because they have a separate paid-for service to handle external email: Azure Email Communication Services (ECS). The external service also runs on top of Exchange Online, albeit in a separate instance.
Now message center notification MC1243552 announces the end of a very long preview period for HVE and imminent general availability for late March 2026 (Microsoft 365 roadmap 382633). Although HVE is a pay-as-you-go service, Microsoft won’t begin to bill customers until a “free promotional period” finishes in May 2026.
High-Volume Email and LOB Applications
Apart from targeting internal groups that generate and send large amounts of emails (think thousands of messages daily), High-Volume Email is also designed to accommodate line of business applications that need to send email as part of their processing. The PowerShell code to use the Send-MailMessage cmdlet to send messages via a HVE mailbox is like the steps needed to send messages from a user or shared mailbox.
For now, HVE supports basic authentication for SMTP submission. This is an important factor for LOB applications where minimal code changes are desirable. However, Microsoft says that they will remove this facility in September 2028. This date is later than the plan to deprecate basic authentication for SMTP AUTH outside HVE, where Microsoft has started the process to phase out these connections and will announce a final drop-dead date in the second half of 2027.
Eventually, all client submissions of email to Exchange Online will be required to use OAuth. As many customer comments to Microsoft’s posts about HVE attest, manufacturers of devices like printers and scanners are not rushing to update their devices to support OAuth submission. This factor might influence the final deprecation date of basic authentication for SMTP AUTH.
The Costs of High-Volume Email
Microsoft hasn’t announced pricing for HVE yet. It’s possible that HVE will use much the same price structure as for ECS, where sending 1 million messages of an average 0.2 MB size costs $274. I’d expect to see pricing details soon, and certainly well before Microsoft turns on charging in May 2026.
Setting Up High-Volume Email
Documentation for HVE is online. HVE is not terribly complicated, and tenants should not face difficulties setting it to send messages, at least not with test scripts. Making sure that the HVE accounts can use basic authentication with SMTP AUTH is usually the biggest issue, including when conditional access policies are involved like the Microsoft-managed policy to block legacy authentication. This (and potentially other) policy must be edited to exclude the HVE accounts. If not, any attempt to send mail with HVE will fail with:
Failed to send email to with error The SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.7.57 Client not authenticated to send mail. Error: 535 5.7.139 Authentication unsuccessful. The organization configuration does not allow this authentication request. Visit https://learn.microsoft.com/en-us/azure/active-directory/develop/reference-error-codes for more information. Error code: AADSTS53003
Updating apps and devices always pose additional challenges. If your organization has a need to send large numbers of internal messages on a regular basis, try using HVE to see if it works well for you. Do so before May and you won’t be charged.
Message Rate Limits Exist for Good Reasons
I have heard from many customers about their unhappiness with the standard limits that stop Exchange Online mailboxes being able to handle sending thousands of messages daily. Those limits exist to stop spammers using Exchange Online as a platform, either by spinning up test tenants or after compromising customer tenants. Charging for ECS to send large amounts of external email makes sure that spammers won’t use it, just like they won’t use HVE because spammers are not too interested in internal recipients.
If you find that High-Volume Email doesn’t meet your needs, many third-party email services are available. Microsoft will point to the integration of HVE with other Microsoft 365 components as its major advantage. However, if a solution can’t do what you want, no amount of integration can help.
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