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 Office 365 environment. When you send emails via an SMTP client (e.g. Mozilla Thunderbird) and use CodeTwo Email Signatures for Office 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 geolocation of your CodeTwo services. Follow the steps below.

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

    and click Next to proceed.

Fig. 1. Adding a new connector in EAC.

  1. Provide a name for your connector (e.g. Inbound Connector for SMTP) and click Next.
  2. When asked for email identification, select the second option and click the plus (+) button (Fig. 2.). Provide the IP address of your CodeTwo service geolocation and click OK.


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

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

  1. Click Next and save your settings. The new inbound connector will be added to the list (Fig. 3.). As soon as the changes propagate in your Office 365 (it might take up to a few hours), your usual mail flow should be restored.

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