Patch by Alain Williams (plus minor tweaks).
Bugzilla 880.
+PP/18 OpenSSL made graceful with empty tls_verify_certificates setting.
+ This is now consistent with GnuTLS, and is now documented: the
+ previous undocumented portable approach to treating the option as
+ unset was to force an expansion failure. That still works, and
+ an empty string is now equivalent.
+
Exim version 4.80.1
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