From what I can see, it just appears to be a standard email client. So I don’t know why you would use this over something that’s open source. And with something like Proton Mail, this would not work if you are a user of that service.

It looks like it has exactly the same pitfalls as every other proprietary email client.

Not that open source email clients are all much better, because email isn’t very private. Even if you encrypt your messages, a lot of metadata leaks. Most encryption solutions don’t even encrypt the subject line, because it too is stored with the metadata.