Some ISP’s block the default port 25 for SMTP on their networks to prevent or reduce spams but this cause trouble for a end user who cannot send out mail using default smtp port.
In that case you can enable Submission Port from Plesk as this will allow the users to send emails from the submission port 587.
To enable submission port from Plesk follow the below steps ::
- Click on Settings > Mail Server Settings.
- From Preferences tab, check the option for “Enable message submission”.