Is There an Alternative Port for Unauthenticated Microsoft 365 SMTP Relay?
Question details
User needs to know if there is an alternative to port 25 for configuring unauthenticated SMTP relay in Microsoft 365 for applications and printers.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Configuring network printers or applications to send emails via Microsoft 365 SMTP relay without using authentication.
- Observed behavior
- Port 25 is blocked or unavailable, and the user is seeking an unauthenticated replacement port, noting that port 587 requires authentication.
Verify your network firewall settings and check if outbound port 25 is blocked by your Internet Service Provider (ISP) or network administrator before changing configurations.
Unblock Port 25 for Direct Send and SMTP Relay
Since Microsoft 365 requires port 25 for unauthenticated SMTP relay, you must configure your network to allow outbound traffic on this port.
There is no alternative port for unauthenticated SMTP relay in Microsoft 365. Direct Send and SMTP relay methods strictly require outbound port 25.
Review your local router and firewall configurations to ensure outbound port 25 is not actively blocked on your local network.
Reach out to your Internet Service Provider to request unblocking port 25, as many ISPs block this port by default to prevent spam.
Once port 25 is open, configure the Microsoft 365 Exchange admin center connector with the correct static IP address or certificate for unauthenticated relay.
Switch to Authenticated Client Submission (Port 587)
If port 25 cannot be unblocked, you must switch to an authenticated connection using port 587 as an alternative to unauthenticated relay.
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Frequently Asked Questions
Can I use port 465 for unauthenticated Microsoft 365 SMTP relay?
No, port 465 is typically used for older SMTP over SSL (SMTPS) connections and is not supported as an alternative for unauthenticated SMTP relay in Microsoft 365. You must use port 25 for unauthenticated mail routing.
Why does my ISP block port 25 by default?
Many Internet Service Providers (ISPs) block outbound port 25 by default to prevent compromised computers and applications on their network from sending unsolicited spam emails directly to external mail servers.
Does authenticated client submission via port 587 require a licensed Microsoft 365 account?
Yes, when using port 587 for SMTP client submission, the device or application must securely authenticate using a valid Microsoft 365 mailbox that has an active Exchange Online license assigned to it.
