The reason why a Cloud Virtual Machine (CVM) cannot use TCP port 25 to connect to external addresses is primarily due to security concerns and spam prevention measures. TCP port 25 is traditionally used for SMTP (Simple Mail Transfer Protocol), which is commonly employed for sending emails. However, because this port is often targeted by spammers, many cloud service providers and internet service providers block or restrict its use on outbound connections from their networks.
To unblock TCP port 25 for outbound connections on a CVM, you typically have a few options:
Use an Alternative Port: One common approach is to use an alternative port for SMTP, such as port 587 (Submission) or port 465 (SMTPS). These ports are often used for secure email transmission and may not be blocked. You would need to configure your email server or client to use one of these ports instead of port 25.
Apply for Port Unblocking: If you must use port 25, you can apply to your cloud service provider for an unblock. They may require you to provide additional information about why you need this port and could impose certain restrictions or monitoring to prevent abuse.
Use a Third-Party Email Service: Another option is to use a third-party email service that handles email delivery on your behalf. These services typically have their own infrastructure and can manage the complexities of email delivery, including compliance with anti-spam regulations.
For users of Tencent Cloud, if you need to send emails from your CVM, it's recommended to use Tencent Cloud's Email Service. This service provides a reliable and secure way to send and receive emails without worrying about port blocking issues. It integrates with your applications seamlessly and offers features like email templates, tracking, and analytics.
Remember, when dealing with email transmission, it's crucial to ensure that your practices comply with relevant laws and regulations to avoid issues with spam or phishing.