default to the word "system" to access the system default CA bundle.
For GnuTLS, only version 3.0.20 or later.
+JH/06 Verification of the server certificate for a TLS connection is now tried
+ (but not required) by default. The verification status is now logged by
+ default, for both outbound TLS and client-certificate supplying inbound
+ TLS connections
+
+JH/07 Changed the default rfc1413 lookup settings to disable calls. Few
+ sites use this now.
+
Exim version 4.85
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