Is it possible to use another port for SMTP relaying of emails if 25 port is blocked by ISP or server provider?

Question

Is it possible to use another port for SMTP relaying of emails if 25 port is blocked by the ISP or server provider?

Answer

It is not possible to use another port for outgoing connections related to SMTP relaying of emails to other mail servers.

That is so, because even nowadays mail servers worldwide relay emails to each other (for the final local delivery) via port 25, due to which this still remains a universal rule.

Note: Plesk supports StartTLS encryption on port 25 by default.

Additional information

Most commonly ISPs and server providers block only port 25 in the outbound direction on a level above the server itself. There are two options to resolve that situation:

• Contact the ISP or server provider and request to have port 25 unblocked for the specific IP address that your server uses.
• Setup relayhost (smarthost) in postfix in order to use a 3rd mail service for email relaying purposees by following steps from any 3rd-party resource, such as this.

If your ISP or server provider also blocks incoming port 25 for incoming traffic, it will be not possible to use mail service on this server (incoming emails will not reach your mail server at all) and you must contact the ISP or server provider for further assistance.

• For configuring Mail Submission port 587 in Plesk for Windows, please refer to this article
• For configuring Mail Submission port 587 in Plesk for, Linux please refer to this article

Note: Plesk server cannot be configured to use 587 port for outbound SMTP traffic. Either only port 25 or port 25 in conjunction with port 587 can be used. Also, Plesk will still be listening on 25 port alongside with 587 and this cannot be changed.