Office 365 smtp server relay
Published November 1, 2024 · Updated July 20, 2026 · SMTP & Deliverability
The Office 365 SMTP server relay is how your organisation's devices, apps, and servers route outbound mail through Microsoft 365. Understanding which relay path fits your case - and their shared limits - saves a lot of frustration. Here is the overview.
The relay paths at a glance
| Path | Server | Best for |
|---|---|---|
| SMTP AUTH | smtp.office365.com | single app |
| Direct send | your MX endpoint | internal only |
| Connector relay | MX + static IP | devices, external |
Shared ceiling: every path runs on Microsoft 365's infrastructure and its recipient caps. None is built for marketing volume.
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Frequently asked questions
What is the Office 365 SMTP server for relay?
smtp.office365.com (port 587) for authenticated submission, or your MX endpoint for direct send and connector relay.
Which Office 365 relay should I use?
SMTP AUTH for a single app, direct send for internal-only mail, and a connector for devices sending externally by static IP.