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.
With the release of Plesk v8.3, you can configure your mail message submission port through Plesk CP to protect your server against unauthorized mail relaying or injection of unsolicited bulk mail.
Select the Enable message submission box on the Plesk->Server->Mail page to allow your customers to send email messages through port 587. Enabling the "Mail Submission" port does not usually affect the 25 (SMTP) port; it is simply an additional way to prohibit unauthorized users from sending mail. If you set the relaying mode to "closed" on the Server->Mail page in Plesk CP, mail will only be received on port 25. Notify your customers that they need to choose port 587 for outgoing SMTP connections in their email program settings, and be sure to allow connections to this port in your firewall settings.
Read more info in official documentation. Configuring Server-Wide Mail Settings