Anti-virus products are fairly reliable at treating an attack that has made it onto your network, but they aren't capable of preventing that attack from reaching you in the first place. This leaves a vulnerability in your defences, not just to invasive attacks (like viruses or malware), but also to the types of non-invasive, such as spear-phishing attacks.
Fantastic Cloud Services' Email Security service acts as a barrier, sitting in front of your email server or cloud-based account, filtering and scanning emails before they hit your inbox. Any threat, whether it be a malicious attachment containing malware or ransomware, or an embedded link to a credential stealing website, is scanned in order to identify it. If the email is found to be harmful in anyway, it is flagged up and the user notified.