Requirements for Office 365 SMTP relay

  1. Static IP address or address range: Most devices or applications are unable to use a certificate for authentication. To authenticate your device or application, use one or more static IP addresses that are not shared with another organization.
  2. Connector: You must set up a connector in Exchange Online for email sent from your device or application.
  3. Port: Port 25 is required and must not be blocked on your network or by your ISP.
  4. Licensing: SMTP relay doesn't use a specific Office 365 mailbox to send email. This is why it's important that only licensed users send email from devices or applications configured for SMTP relay. If you have senders using devices or LOB applications who don't have an Office 365 mailbox license, obtain and assign an Exchange Online Protection license to each unlicensed sender. This is the least expensive license that allows you to send email via Office 365.

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