Smoothwall requires an SMTP server to be configured on the Output settings page to be able to send out the various types of notifications that the monitor and reporting system manages.
With a local SMTP server or relay present, this doesn't pose much of a challenge. Point the Smoothwall system to that internal SMTP server or relay.
When using cloud based email domains, like Office 365 or G-Suite, additional steps needs to be taken in order for the mails to be processed correctly.
Here are links to knowledge base articles for both types, to show what needs to be done in order to allow for SMTP access to the mail engines used.