Please see SPL-138647 in the release notes to determine what SSL/TLS version and cipher suites your e-mail server supports:
http://docs.splunk.com/Documentation/Splunk/6.6.0/ReleaseNotes/Knownissues
Alternatively, if security is not a concern, you can also revert to the 6.5.x configuration:
$SPLUNK_HOME/etc/system/local/alert_actions.conf
[email]
sslVersions = *,-ssl2
cipherSuite = TLSv1+HIGH:TLSv1.2+HIGH:@STRENGTH
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In addition to the changes to server.conf, you also have to delete $SPLUNK_HOME/splunk/var/run/splunk/_raft on each of your search heads. You can then restart and run "bootstrap shcluster-captain" again.
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