Direct Send vs SMTP relay vs HVE: which to choose

Updated: July 2026

To have an application send mail through Microsoft 365, several native mechanisms exist. None covers every need. Here is what each does, its limits, and where a relay like fluctuat fits.

SMTP AUTH (authenticated relay)

The application connects to smtp.office365.com with a credential. Simple, but basic authentication is going away in 2026 (rejected since April, off by default at year end), and it drops no copy on the sender side.

Direct Send

Sending without authentication to your own domain, via yourdomain-com.mail.protection.outlook.com. Handy for internal notifications, but no copy in Sent Items, more fragile deliverability to external recipients, and continued hardening on Microsoft's side.

High Volume Email (HVE)

Microsoft's bulk-send mechanism, now geared to internal use and subject to its own limits. Again, no copy on the sender side.

Where fluctuat fits

fluctuat is a relay that speaks modern authentication to Microsoft 365 on your behalf. Your applications keep SMTP, with no recoding. The message goes out through the Graph API, within your own tenant, as the real sender, with a copy in its Sent Items, and a durable queue that never silently drops mail. It is the only one on the list that combines modern authentication, real sender, sender-side copy, and zero recoding.

Create an account, first send in ~15 min

Free to start, no card. A Sovereign edition is available for sending that never transits through Dizzus.