Knowledge Base

Emails sent via SMTP are not routed correctly when you use a smart host service


You use smart host (mail relay) services (e.g. Barracuda) for security or anti-spam protection in your Microsoft 365 (Office 365) environment. When you send emails via an SMTP client (e.g. Mozilla Thunderbird) and use CodeTwo Email Signatures 365, your messages are not routed correctly, e.g. they do not pass through your smart host or they are not delivered to recipients. The problem does not exist if you use Outlook (the MAPI protocol) to send emails.


If your environment is similar to the one described above, emails sent via the SMTP protocol will be routed by EOP to CodeTwo services but they will not be passed to your smart host service.

To fix this problem, you need to create an additional inbound connector that accepts messages if the sender's IP matches the Azure region of your CodeTwo services. Follow the steps below.

  1. Sign in to your Microsoft 365 admin center.
  2. Go to Admin centers Exchange to access your Exchange admin center (EAC).
  3. Select Mail flow > Connectors from the left menu. Apart from the default CodeTwo connectors, you can probably see the outbound connector responsible for your smart host service (Fig. 1.).
  4. Click the Add a connector button (Fig. 1.) to create a new inbound connector. A configuration wizard will open. Select the following mail flow scenario for the new connector:
    Connection from: Your organization's email server
    Connection to: Office 365

    and click Next to proceed.

Adding a new connector in EAC.
Fig. 1. Adding a new connector in EAC.

  1. Provide a name for your connector (e.g. Inbound Connector for SMTP), leaving other options unchanged and click Next.
  2. In the Authenticating sent email step of the wizard, select the second option (Fig. 2., Item 1), provide the IP address of your CodeTwo service region in the text box (Fig. 2., Item 2), and click the plus (+) button (Fig. 2., Item 3) to add the IP address.


To get the IP address of your CodeTwo Azure region, see this article.

You need to provide the IP for the Azure region of your CodeTwo services.
Fig. 2. You need to provide the IP for the Azure region of your CodeTwo services.

  1. Click Next, and then, in the last step of the wizard, click Create connector and Done. The new inbound connector will be added to the list (Fig. 3.). As soon as the changes propagate in your Microsoft 365 (it might take up to a few hours), your usual mail flow should be restored.

The new inbound connector for emails sent via SMTP.
Fig. 3. The new inbound connector for emails sent via SMTP.

See also:

How to configure smart host (mail relay) services to work with CodeTwo software

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