SMTP
Are you looking to send emails from your server? This can be either you want to setup an email service or you have an application you installed that sends emails.
Many VPS providers disable port 25 by default, which means if you want to send outbound emails, you may need to contact your VPS provider and request port 25 to be unblocked.
We recommend using a service you can connect your apps with to manage sending emails for you, such as MailGun.