Please refer to the following steps to enable port 587 to relay your emails from Plesk.
- Log in to Plesk Panel using admin credentials.
- Click on the Tools & Settings >> Mail Server Settings.
- At Mail server settings find the option called Enable SMTP service on port 587 on all IP addresses and tick the check box.
- Finally, click on the OK button and allow port 587 in your server firewall without fail.