Office 365 smtp relay
Published November 3, 2024 · Updated July 20, 2026 · SMTP & Deliverability
Sending application or device email through Microsoft 365 is common — printers, line-of-business apps, and scripts all need a way to relay mail. Office 365 offers a few ways to do it, each with different limits and trade-offs. Here is how Office 365 SMTP relay works, which method fits your case, and where it stops being the right tool.
The three ways to relay through Office 365
| Method | How it authenticates | Best for |
|---|---|---|
| SMTP AUTH client submission | Mailbox login (port 587) | Apps sending as one user |
| Direct send | None (to your own domain) | Internal-only mail |
| Microsoft 365 SMTP relay | Connector + static IP | Devices relaying to external |
How to set up SMTP AUTH client submission
- Ensure SMTP AUTH is enabled for the mailbox (it is disabled by default on newer tenants).
- Use host
smtp.office365.com, port 587, STARTTLS. - Authenticate with the mailbox's email address and password (or an app password with MFA).
- Send — the app now delivers as that user.
Bulk sending on a dedicated SMTP server
Keep Microsoft 365 for business mail; route marketing and high-volume sends through a dedicated IP with unlimited throughput. From $39.99/mo.
Frequently asked questions
What is the Office 365 SMTP server and port?
Use smtp.office365.com on port 587 with STARTTLS for authenticated client submission. Enable SMTP AUTH on the mailbox first, since it is off by default on new tenants.
Can I send bulk email through Office 365?
Not really — Office 365 caps recipients per day (around 10,000) and is built for business mail, not marketing. For bulk, use a dedicated SMTP server on its own IP.
Why is SMTP AUTH disabled in my tenant?
Microsoft disables it by default for security. An admin can enable it per-mailbox in the Microsoft 365 admin center or via PowerShell.