How to Setup smtp relay office 365
Published October 20, 2024 · Updated July 16, 2026 · SMTP & Deliverability
Need to set up an Office 365 SMTP relay so an app can send email? For most single-app scenarios the simplest route is SMTP AUTH client submission - authenticate as a mailbox and send. Here is the setup, step by step.
SMTP AUTH setup
- Confirm SMTP AUTH is enabled for the mailbox (it is off by default on newer tenants - an admin enables it).
- Use host
smtp.office365.com, port 587, STARTTLS. - Authenticate with the mailbox address and password (or an app password with MFA).
- Send a test and confirm delivery.
The limit: client submission sends as one mailbox and inherits Office 365's daily recipient cap (~10,000). Fine for app notifications - not for marketing blasts.
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Frequently asked questions
How do I set up SMTP relay in Office 365 for an app?
Enable SMTP AUTH on the mailbox, then send via smtp.office365.com port 587 with STARTTLS, authenticating as that mailbox.
Why is SMTP AUTH disabled?
Microsoft disables it by default for security; an admin can enable it per-mailbox in the admin center or via PowerShell.