How to Exchange online smtp relay

Published November 1, 2024 · Updated July 17, 2026 · SMTP & Deliverability

Exchange Online — the email backbone of Microsoft 365 — can relay mail from your apps, printers, and internal systems. But the method you choose, and the limits you inherit, make a big difference. Here is how Exchange Online SMTP relay works, how to set it up, and the point where a dedicated SMTP server becomes the smarter route.

Your options for relaying through Exchange Online

Method	Sends to external?	Notes
SMTP AUTH submission	Yes	Sends as a licensed mailbox
Direct send	Your domain only	No auth, internal use
Connector relay	Yes	Needs a static public IP

How to set up SMTP AUTH relay

Confirm the mailbox has SMTP AUTH enabled (off by default on new tenants).
Use smtp.office365.com, port 587, STARTTLS.
Sign in with the mailbox address and password (or an app password if MFA is on).
Send a test to an external address and confirm delivery.

The hard limit: Exchange Online enforces a recipient-rate limit (about 10,000 per day) and is built for business correspondence, not marketing volume. Bulk campaigns through it risk throttling and can dent your whole tenant's reputation.

Keep Exchange for business mail

Route bulk sends through a dedicated SMTP

Dedicated IP, unlimited volume, and automatic SPF/DKIM/DMARC — the throughput Exchange Online isn't built for. From $39.99/mo.

See unlimited SMTP plans →See all plans

Frequently asked questions

What is the Exchange Online SMTP server?

smtp.office365.com on port 587 with STARTTLS, used for authenticated client submission from apps and devices.

Can I send marketing emails through Exchange Online?

It is not recommended — daily recipient limits and business-mail design make it a poor fit. Use a dedicated SMTP server for marketing volume.

Why is my Exchange SMTP relay being throttled?

You are likely brushing the recipient-rate limit or triggering anti-abuse protections. Move high-volume sending to a dedicated SMTP with its own IP.