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Microsoft 365 does not support username and password authentication to an external smart host. Contact mySMTP support to arrange IP-based authorization before creating the connector.

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Before you start

Have these details ready:
Use the exact relay hostname from your setup email. relayxx.mysmtp.com is only an example.

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Create the outbound connector

1

Open the Exchange admin center

Go to the Exchange admin center and sign in with a Global Administrator or Exchange Administrator account.
2

Start a new connector

  1. Go to Mail flow > Connectors.
  2. Click Add a connector.
  3. Select Connection from: Office 365.
  4. Select Connection to: Partner organization.
  5. Click Next.
3

Name the connector

Enter a name such as mySMTP outbound and an optional description. Click Next.
4

Choose when to use the connector

Select Only when email messages are sent to these domains. Click the + icon, enter * as the domain, and click OK. Click Next.
5

Set the smart host

  1. Select Route email through these smart hosts.
  2. Click the + icon and enter your relay hostname (e.g. relayxx.mysmtp.com).
  3. Click Save, then Next.
6

Require TLS

  1. Select Always use Transport Layer Security (TLS) to secure the connection.
  2. Choose Issued by a trusted certificate authority (for example, DigiCert or GlobalSign).
  3. Click Next.
7

Review and create

Review the settings and click Create connector.
Microsoft 365 outbound connectors do not support username and password authentication to an external smart host. Authentication is handled by the sending IP address or a certificate. Contact mySMTP support to authorize Microsoft 365’s outbound IP ranges for your account.

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Create the connector with PowerShell

If you prefer PowerShell, connect to Exchange Online PowerShell and run:
Replace relayxx.mysmtp.com with your actual relay hostname.

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SPF record

Add both Microsoft 365 and mySMTP to your domain’s SPF record. A typical record looks like:
If you already have an SPF record, add the includes without removing existing entries. Too many DNS lookups can cause SPF failures. Use the Deliverability Toolbox to check your SPF record.

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Test the connection

  1. Send a test email from a Microsoft 365 mailbox to an external address, such as a Gmail account or your own.
  2. Confirm the message arrives. In the received message headers you should see it passed through your mySMTP relay.
  3. If the message does not arrive, check the Message trace in the Exchange admin center under Mail flow > Message trace.

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Troubleshooting

  • Connector not used: Make sure no other outbound connector with a higher priority or matching rule is intercepting the mail. Microsoft 365 evaluates connectors in order.
  • TLS handshake fails: Confirm the connector requires TLS and that your network allows outbound connections on port 587.
  • Relay access denied: Verify that mySMTP support has authorized Microsoft 365’s outbound IP ranges for your account.
For more error codes, see Common SMTP errors.