You already got a CA in place. Use your CA to generate a new certificate for your mail server. Add that certificate and your Root CA to your mail server setup.
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Use openssl
s_clientto connect to your mail server -
verify that you set the certificate for incoming and outgoing connections
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send an email with starttls using a MUA.
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Use a MUA to send an email to your HdM Account using your mail server system as a relay.
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both connections should be secured via TLS. Look in your mail.log to verify this
