We strongly recommend using a dedicated email service. Email server setup and maintenance is very difficult even for experienced system administrators, and getting any part of the complex required email setup wrong means your email won't be delivered, or worse, delivered erratically.
The following are template configurations for email service providers who offer generous free plans that work for most communities.
Please note that in any email provider, you must verify and use the subdomain, e.g. discourse.example.com. If you verify the domain only, e.g. example.com, mail will not be configured correctly.
Enter these values when prompted by ./discourse-setup per the install guide:
SparkPost — 100k emails/month
SMTP server address? smtp.sparkpostmail.com
SMTP user name? SMTP_Injection
SMTP password? [Any API key with Send via SMTP permission]
SendGrid — 40k emails on a 30 day trial
SMTP server address? smtp.sendgrid.net
SMTP user name? apikey
SMTP password? [SendGrid API Key]
We recommend creating an API Key instead of using your SendGrid username and password.
Mailgun — 10k emails/month
SMTP server address? smtp.mailgun.org
SMTP user name? [SMTP credentials for your domain under domains tab]
SMTP password? [SMTP credentials for your domain under domains tab]
Mailjet — 6k emails/month
Go to My Account page and click on the "SMTP and SEND API Settings" link.