mail unexpectedly quit and my emails were gone

iCloud is synchronized with your devices. Sounds like the malware deleted your email and then your Mac sync'd to iCloud and the emails are gone. This is why you should have a backup plan.

The drop dead easiest backup available is Time Machine. It's as simple has finding a drive that's 2-3 times larger than your internal Mac drive and connecting it to your Mac and macOS will popup and ask if you wish to use it as a Time Machine. All you have to do is turn in ON. It will schedule a full backup to start within about 15 minutes. A full copy of your drive is copied to the disk and then when that finishes it will update the backup every hour with interval backups. Time Machine will then merge hourly backups into daily backups and daily backups into weekly backups and weekly into monthly. When the Time Machine drive is not attached, the Mac will snapshot backups on APFS and when you connect the Time Machine drive those snapshots will be sent to the backup and the snapshots removed.

If you had a Time Machine backup you would be able to back in time and restored your email from that time. Once restored, that email will sync with iCloud and subsequently to all your other devices.

Another choice is to clone your drive using software such as Carbon Copy Cloner or SuperDuper. Typically you only need a drive as large as your internal drive or a bit larger. Then you manually run the clone process which makes a complete copy of the current state of your disk. Carbon Copy Cloner added APFS snapshot support and you can manage the snapshots as well. This speeds up the backups as only the snapshots are cloned.

Other backup solutions would be 3rd party cloud solutions provided by companies such as Backblaze where you subscribe to their backup service and install the software and it will backup to the cloud. This has the advantage of being offsite so if your home / office is destroyed by a fire, flood, or other natural disaster your data is still up there in the Cloud.

What is a complete backup strategy? All of the above and you rotate multiple external backup drives offsite. That would be the ultimate level of protection. Having multiple backups is recommended so that means having alternatives to Time Machine that are run in parallel. But that is a lot of hassle for most people. So you need to figure out what your most critical data is and come up with a bare bones method to backup that critical data at the minimum. In many cases, years of valuable data is far more valuable than the computer hardware.

At the very least, you should look into using Time Machine. It's built-in to macOS and is a no brainer. It runs automatically and you don't need to think about it. Personally, Time Machine has saved myself and several family, friends, and coworkers from absolute disaster many times over the years. I've yet to find anything else that comes close to it's ease of use. That being said, sometimes, Time Machine can have problems. Such as when the disk is full and you see a warning to that effect. Your option is to replace the Time Machine drive with another one as you have reached the limit where it can continue adding to the backup history. Or you can allow Time Machine to remove the oldest backups to make room for the current backups. If you keep the old drive and add a new one you can re-attach the old drive and recover data from further back in time. But if you haven't used those old files that were deleted that are taking space on your Time Machine and you have a large enough drive to retain a rather long history then it might not be a problem losing those files.

Both iOS / iPadOS do not allow any rogue code to run. Only Apps from the App Store. Yes, there are vulnerabilities but providing you keep the mobile devices up to date the risks are extremely low. I've only heard of nationstate hacking going on with Apple mobile devices. So dissidents, authors, news media, etc. being specifically targeted by government hackers.

With Macs you have more freedom and with it comes greater risk of malware. You need to really be on your toes with both phishing via email and text messages or any other messaging apps. You should only install software from the Mac App Store and if you do need software that is not on the App Store that you download it directly from the manufacturer / developers website that you manually navigated to yourself. That software should be signed and notarized and if it's not, you should seriously question its legitimacy. When you run software that has not been signed nor notarized you will see a warning from Apple. Do not become accustomed to just allowing it to run. Most developers have done the right thing and both signed an App and submitted it to Apple so they can notarize it. Notarization means that Apple has verified it is not including malware.

Be wary of free games not distributed via the App Store as many include adware as a source of income. Avoid those download sites with shareware as they also bundle adware as a revenue stream. Most things that are free are finding some way to generate revenue. Even on the Apple App Store there have been Apps with ridiculous subscription models or in App purchases that have taken advantage of people. Apple has been cleaning that mess up lately.

The big one is to never pirate software. It's just too juicy a target for malware. Sure you might be getting some commercial software for free but it's highly likely to include a malicious payload coming along for the ride.

Running an ad blocker is highly recommended. I've come across many rogue ads even on mainstream websites that auto downloaded a bit of malware. Advertising networks accept small outfit ads and feed them through the network. Many are not doing the due diligence of testing the advertisements. Now and then some rogue advertisement delivers a download that might run automatically. Other ads are fake security warnings about your computer being hacked, etc. Do not click on these as they will either deliver malware or try to fool you into installing fake security software as you've discovered with MacKeeper.

Beware of scammers calling your phone claiming to be Apple or Microsoft and telling you that you are infected. Do not establish a remote control session with these scammers. Do not engage with them at all. Just hang up.

Why is malware / adware such a massive problem? Well it costs the scammers next to nothing and the returns are massive. Millions of people around the world are fooled continuously. Billions of dollars are stolen worldwide every year. Hundreds of millions in the USA alone.

It is recommended to be far more skeptical about software and to think twice before installing any software. It's now a zero trust world. The scammers are out there and there is massive financial incentive for them to scam as many people as possible in every conceivable way.