Exchange Connection Problems

Fix Microsoft 365 SMTP Relay Error 4.5.3: Recipients Belong to Multiple Tenants

Emma Brown Oct 7, 2026 868 views

Question details

The user needs to resolve an SMTP relay failure where emails are rejected because the transaction includes recipients in different Microsoft 365 tenants.

How to Fix Microsoft 365 SMTP Relay Error 4.5.3 (Recipients Belong to Multiple Tenants)

Product: Microsoft 365 Exchange Online
Device & OS: not provided
Scenario: Sending an email message through Microsoft 365 SMTP relay (Port 25) that includes both external recipients (in the To field) and internal recipients (in BCC).
Observed behavior: The server rejects the email, returning an 'Insufficient system storage' error with the response code '4.5.3 Recipients belong to multiple tenants'.

Before you start

Verify your organization's Exchange Online accepted domains and connectors, and ensure you have access to modify your application's SMTP sending logic.

Solution 1Recommended

Split the Message by Destination Tenant

Because Microsoft 365 SMTP direct relay natively restricts routing a single transaction across multiple tenants, separating the recipients into distinct messages is the most reliable resolution.

Exchange Online does not provide a tenant-level setting to override this security restriction for Port 25 direct send relays. The application generating the email must handle the recipient separation.

Modify Application Logic

Access the source code or mail configuration panel of the application generating the email.

Separate the Recipient Lists

Configure your mailing script to process internal (your verified domain) and external (different tenant) recipients in separate arrays.

Generate Distinct Transactions

Set the application to trigger a distinct SMTP send request for the external 'To' recipients, and a completely separate SMTP request for the internal 'BCC' recipients.

Solution 2

Switch to SMTP Client Submission (Port 587)

If separating the messages is not feasible, using authenticated SMTP Client Submission bypasses the specific multi-tenant restrictions of Port 25 direct relays.

Solution 3

Implement a Third-Party Application Relay

A dedicated outbound third-party relay service can seamlessly handle complex cross-tenant routing without triggering Microsoft 365's specific direct-send restrictions.

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Frequently Asked Questions

Why does Microsoft 365 block SMTP direct relay to multiple tenants?

Microsoft imposes this restriction on Port 25 unauthenticated direct routing to prevent spam, unauthorized spoofing, and abuse of its server infrastructure. Allowing a single transaction to cross tenant boundaries poses a security risk.

Can I fix Error 4.5.3 by adding the external domain to my Exchange accepted domains?

No. You can only add domains to your accepted domains list if your organization actively owns and controls them. Adding an external company's domain is not possible or permitted.

Does this multi-tenant error happen when sending emails normally from Outlook?

Generally, no. Error 4.5.3 typically occurs only during unauthenticated SMTP relay (using Port 25) from background applications, scanners, or servers. Standard user emails sent via Outlook use authenticated client submission protocols.

What is the difference between direct send and SMTP client submission?

Direct send uses Port 25 and IP authentication to send emails strictly to recipients within your own organization. SMTP client submission uses Port 587 with mailbox credentials, allowing emails to be routed to internal and external recipients.

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