I’m a Apple Mail user but from time-to-time am tempted to dabble with third-party mail apps. Why? Not exactly sure. Apple Mail does the “basics” right, but certainly does not include features like snooze, send later, etc. like Airmail and Spark, amongst others. While these apps offer fantastic features and user experiences, something in the back of my mind says “Won’t the developer have access to my emails in some shape or form?” and that’s always enough to bring me back to Apple Mail. Is my logic flawed? Should I use these third-party apps without worry?
Don't use any software from AD companies (Microsoft, Google, Facebook). Use Firefox email client or even Apple.
That’s exactly why I only use the app from the email provider. Email service provider like gmail already has your emails. If you are using any other app even apple mail, you are giving away your emails to a new entity. I would be careful.
If you’re using a popular mail client, there’s a good chance some hobbyist somewhere is snooping the packets of that app, or messing around with the binary. I’d be more careful with relatively unknown mail clients