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Why I ditched Gmail for Proton Mail
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
You can switch MX records but not necessarily your mail storage. You need IMAP for that, and IMAP with Proton currently requires jumping through some hoops and it may be discontinued in the near future.
They've never given any guarantees regarding IMAP and they actually seem to consider it a negative so that remains a dubious point with me.