For security purposes, Alibaba Cloud disables outbound access through TCP port 25. That is, on an Alibaba Cloud Elastic Compute Service (ECS) instance, you cannot access external addresses through TCP port 25.
If outbound access through TCP port 25 is disabled, you may be unable to connect the Simple Mail Transfer Protocol (SMTP) server of a third-party mail service provider through this port to send emails. To send emails on an Alibaba Cloud ECS instance, we recommend that you use port 465 provided by a third-party mail service provider.
In special scenarios where you must use TCP port 25 on an ECS instance for external access, apply to enable TCP port 25 on the Alibaba Cloud Security Control console.
- Log on to the Alibaba Cloud Security Control console.
Note Move your cursor to the account icon in the upper-right corner of the Alibaba Cloud console, and click Security Console to go to the Alibaba Cloud Security Control console.
- Choose Apply to open Port 25.
. On the page that is displayed, click Notice Before applying, you must agree to use TCP port 25 only for communicating with and sending emails through a third-party SMTP server. If Alibaba Cloud detects that you are using the IP address you requested to directly send emails over SMTP, Alibaba Cloud has the right to permanently disable TCP port 25.
- Select an IP address type and enter the IP address from which you want to allow external access through TCP port 25. Associate the IP address with the target domain name, and click OK.
- Select Agreed "Terms Governing the Use of Port TCP 25" and click Next.
- Fill out the application form and describe the use of TCP port 25 in detail.
- Click OK to submit your application.
Note In normal cases, Alibaba Cloud will review your application based on the reason specified within one work day after it is submitted, and will notify you of the result by email.