just wanna point out that using a secure mail service won’t help if all your emails are sent to Gmail/Google first and then forwarded to your secure mail
What protocol are you using to connect to your mail service and what third-party apps do you need it to be compatible with? POP, IMAP, SMTP, or those protocols with various SSL/TLS implementations? Thunderbird will likely handle anything you throw at it, but I’m at a loss when you start talking about third-party apps without more info.
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