If you're talking about CrashPlan backing those files up, then it's possible yes. Apple mail has many small constantly changing parts, this can cause issues ...
Inability to restore files in the event of ... a disaster
Well, first, that was a horrible explanation from the CrashPlan folks. However, near as I can tell, their backup service suffers from a common problem with a lot of backup software out there: the inability to backup metadata, or the inability to back up metadata such that it will be useful once restored. This is what originally catapulted SuperDuper! to popularity when OS X was young, because just about no backup programs at the time could handle OS X's metadata correctly, but SuperDuper! did so with aplomb.
For those who don't know what metadata is, a computer has programs, and it has files...most backup programs can backup those easily. Metadata is what is on your computer in addition to apps and files. It is literally "data about data". It's preferences files, and databases, and caches, and stuff like that. If you restore a backup that doesn't handle metadata well, you'll get back all of your apps and files, but it may take weeks to restore everything so that it works properly; and some apps that store data internally may simply have all their data hosed.
In 2020 any backup product that doesn't handle all of your Mac's metadata well is simply unacceptable and inexcusable. The above situation is easily enough for me to warn people far away from CrashPlan.
On the other hand, I
already recommended against using cloud-based backup services as part of most backup plans. (Okay, yes, I can think of some situations where cloud-based backup might be worthwhile to have in a multi-tiered backup plan when you have extremely valuable data.) I won't re-hash the security argument about putting your sensitive data on someone else's server (you've all seen the articles about your own government spying on you), or that putting your data in someone else's hands could be a bigger risk than you might think. (Recently LegalWorks, a HUGE cloud-based application for attorneys had their server hit by ransomware, decimating all of the client files for a large number of law firms.)
Mostly I don't recommend online backup services because of what a huge bottleneck the Internet is. Most ISP's these days are capping the amount of data you can transfer daily, and they throttle your access speeds if you transfer too much data all at one time.
Many broadband ISPs have monthly data caps in the 300 GB to 1,000 GB range. Also most ISPs offer much lower upload speeds than download speeds. Typical Comcast 50 megabit/sec service is about 10 megabit/sec upload. To back up a 3TB iMac 27 would take about a month.
When a failure happens and you must recover, time is often of the essence because you need to resume your work. Assuming you patiently waited a month to back up your 3TB iMac 27 to the cloud, if your disk crashed it would take six days to recover over a 50 megabit/sec connection, at best.
I think that it just makes WAY more sense to backup to an external hard drive. It's cheaper in the long run, you can keep your data offline for security, you can have multiple redundant backups for as much safety as you like (some can be stored off-site, if you like), and they can be utilized INSTANTLY in the event of a catastrophe. A clone backup made with a good cloning program will backup all of your metadata so well that you won't even be able to distinguish between booting from your clone backup and your original internal hard drive.