Is it possible to use another port for SMTP connection if 25 port is blocked by ISP?
No, it is not possible to use another port instead 25 for SMTP connection.
Usually, ISP blocks only port 25 for outgoing direction. There are two options:
- Contact ISP and ask to unblock port 25 for outgoing traffic.
- Setup relayhost (smarthost) in postfix following steps from any 3rd-party resource such as this.
If ISP also blocks incoming port 25 for incoming traffic, it will be not possible to use mail service on this server. Contact ISP for assistance.
- For configuring Mail Submission port 587 in Plesk for Windows please refer to the article
- For configuring Mail Submission port 587 in Plesk for Linux please refer to the article
Note: Plesk server cannot be configured to use 587 port for outbound SMTP traffic. Either only 25 or 25 in conjunction with 587 can be used. Also, Plesk will still be listening on 25 port alongside with 587.