Everyone is now, or should be now aware of cryptolocker threats to your business. The Youtube clip below from Kevin Mitnick (an ex computer hacker) very nicely demonstrates how easy it is to compromise your systems and data.
Even with Local Anti Virus, Microsoft ATP, Multi Factor Authentication and link checking this would still be able to encrypt your emails and files in the cloud. Everyone assumes that their data is backed up and can be restored from an earlier times because their data is in Office 365, this is not the case, Office 365 syncs your data so any emails or files that get encrypted are also synced to Office365.
This is why it is absolutely critical that you data is backed up to an Offline point in time backup so as it can be easily restored if necessary without having to pay any ransom. Paying a ransom is by no means guaranteed to work and doing so puts cash into criminals pockets and your business on the radar of criminals that could then concentrate on you in the future in a more targeted attack.
Office 365 backups are simple to implement and you do not need any hardware in your Office to do them. The backup backs up your data up to 6 times a day, you have your own control panel that allows you to restore files, emails and Teams data whenever you want so if the worst was to happen you can get back up and running very quickly.