For security purposes, Alibaba Cloud blocks outbound traffic transmitted through TCP port 25. Therefore, you cannot use Alibaba Cloud instances to access external addresses through TCP port 25.

Background information

You may not be able to send emails to external addresses by connecting TCP port 25 to the Simple Mail Transfer Protocol (SMTP) server that is provided by a third-party mail service. If you want to use an Alibaba Cloud instance to send emails to external addresses, we recommend that you use port 465 provided by the third-party mail service.

Notice In scenarios where you must use TCP port 25 to establish a connection between an Alibaba Cloud instance and an external address, request to enable port 25 in the Security Control console. Alibaba Cloud will review your application based on the reason you submit and notify you of the result by email.

Procedure

  1. Log on to the Alibaba Cloud Security Control console.
    Note Move the pointer over the account icon in the upper-right corner of the Alibaba Cloud console and click Security Console to go to the Security Control console.
  2. Choose Application > Open Port 25. On the page that appears, click Apply to open Port 25 in the upper-right corner.
    Notice Before you apply, you must confirm and agree that TCP port 25 is used only to connect the SMTP server of a third-party service provider and send emails to external addresses. If you use the IP address that you request to directly send emails over SMTP, Alibaba Cloud shall permanently disable TCP port 25.
  3. Select an IP address type, fill out the application form, and then click OK.
    Set the following parameters:
    • Enter IP: Enter the IP address of the instance for which you want to enable port 25. The IP address must be a public IP address of an Alibaba Cloud instance that belongs to the current Alibaba Cloud account.
    • Enter domain name: Enter the domain name of the mail server for which you want to enable TCP port 25.
  4. Select Agreed "Terms Governing the Use of Port TCP 25" and click Next.
  5. Fill out the application form and describe the use of TCP port 25 in detail.
    Set the following parameters:
    • Type of email to send: Enter the type of emails that you want to send from the server connected to TCP port 25.
    • No. of emails to send per day: Enter the number of emails that you want to send from the server connected to TCP port 25.
    • Include an unsubscribe function:: Select whether the server connected to TCP port 25 provides the unsubscription service.
    • Usage description: Enter the purposes of your application in detail.
  6. Click OK to submit your application.
    Note Alibaba Cloud will review your application based on your reason and notify you of the result by email within one business day.