This, of course, opens up the whole can of worms that traditionally plague arguments about which OS is more secure. In that argument, which is a bigger, more rewarding target? A big company who host 'millions and millions' or a smaller company with fewer users? Of course, neither Microsoft nor Apple are small companies (and that also argues for AV software on both platforms) and are open to attack. Or are companies looking for opportunities to monitize our use for their bottom line.
And again, for email, that relies on how comfortable you feel with either (any) company having that data and how 'secure' they actually are.
That's a logical fallacy. Being bigger and more of a target doesnt mean they arent spending an infinite more amount of money securing it than a little guy. That is like saying you shouldnt use 1Password or Lastpass because they are "bigger" Well no, they spend a TON of money on cybersecurity/monitoring; far more than any little guy or yourself hosting your own password vault could. The fact that you can sync your own 1Password vault yourself over say Dropbox, is yet another point of high vulnerability. That is all that causes, it is not "more secure" because its not in the hands of the big company with the resources to protect it.
That relates directly to this 3rd party email app argument; another point of ingress and what is the most vulnerable point.
Your data is already "somewhere" be it Exchange/outook, gmail, yahoo, etc. It is NEVER 100% foolproof secure iron vault. That is another fallacy. Even with the Apple Mail app your data is in the cloud somewhere still. We cant pretend its ever 100% safe.
But if you asked me who do you trust more MS with decades of experience and a track record in the corporate world, or Edison who has been around in mail since 2016 (formerly Easilydo), with an already HORRIBLE publicly exposed track record for user privacy rights/data usage, its an easy A or B choice. MS is the stronger of the 2 possibl vulnerability spots in my opinion.
There is no exact science here, merely opinions. Everyone can do what they want with their data.