Failed to send notification

my smtp config is:

enabled = true
host = xxx:25
user = xxx
password = xxx
;cert_file =
;key_file =
skip_verify = true
from_address = xxx
from_name = Grafana
ehlo_identity =

bug in the grafana version 5.xx and 6.xx ,have a same error :

lvl=info msg=“Sending alert notification to” addresses=[xxx]
lvl=eror msg=“Failed to send alert notification email” error=“unencrypted connection”
lvl=eror msg=“failed to send notification” logger=alerting.notifier uid= error=“unencrypted connection”

in the grafana version 4.5.2 not found this error。