In order to enable the Mail system you have to go under Settings -> Mail and fill ALL the following fields:

Username: the username for authentication

Email Address: the email address used as From field; if empty is equal to Username.

Password: the password for authentication

SMTP Host: the Hostname of the SMTP server

SMTP Port: the Port number of the SMTP serverĀ 

SMTP Secure: the security type for the connection (SSL, TSL, No secure)

Use SMTP: it is necessary to set it on TRUE. If false, the system doesn't use the SMTP server configured above.

If you use your local mail server for relay (for example POSTFIX or SENDMAIL) please see the file /var/log/maillog in order to check that everything is correct