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Mozilla Thunderbird Challenges Gmail With Its Own Email Service
(www.forbes.com)
This is a most excellent place for technology news and articles.
You can't know that with absolute certainty. Sorry, but if you're using someone elses server for your communications and they're not end to end encrypted, you should just assume that they can and do read your emails, and act accordingly.
Mine is E2EE.
Unless everyone you communicate with have agreed to use the same standard as you, no, it is not.
Do you so it over PGP? Or is it done with ZAE like with Proton?