If SMPT port 25 is not working from your end, kindly enable Submission Port from Plesk.
It's very common that an ISP is blocking the default SMTP port or port 25.
Basically, they are doing it to prevent or reduce spams.
If an end-user cannot send out mail because the default SMTP port is blocked from his/her ISP, you should enable Submission Port from Plesk.
This will allow you to use port 587 for SMTP.
From Plesk control panel:
1. Click on Settings > Mail Server Settings.
2. From Preferences tab, check the option for "Enable message submission".
After that, you should be able to use port 587 for sending out email.
