Sendmail is not capable of doing it in the command line and mailx is a nightmare with email headers. Which leaves us only with Curl as an option.
Curl at first seemed to work perfectly, but later we noticed that the -n option which points to the .netrc file isn´t read.
define command {
command_name host-notify-by-email-html
command_line /usr/bin/perl $USER1$/
notify-by-email.pl \
--template=$USER4$/etc/mail-templates/notify-by-email.host.html.basis.tpl \
--mail='/usr/bin/curl --ssl-reqd smtp://
example.com --mail-from
example.com --mail-rcpt "$CONTACTEMAIL$" -n --upload-file "/dev/stdin" --silent' \
-o BASEURL="http://`hostname -f`/$USER3$/" \
-o NOTIFICATIONTYPE='$NOTIFICATIONTYPE$' \
-o NOTIFICATIONCOMMENT='$NOTIFICATIONCOMMENT$' \
-o HOSTNAME='$HOSTNAME$' \
-o HOSTALIAS='$HOSTALIAS$' \
-o HOSTSTATE='$HOSTSTATE$' \
-o CONTACTEMAIL='$CONTACTEMAIL$' \
-o HOSTADDRESS='$HOSTADDRESS$' \
-o SHORTDATETIME='$SHORTDATETIME$' \
-o SERVICEDESC='$SERVICEDESC$' \
-o SERVICESTATE='$SERVICESTATE$' \
-o SERVICEOUTPUT='$SERVICEOUTPUT$' \
-o LONGSERVICEOUTPUT='$LONGSERVICEOUTPUT$' \
-o SERVICEPERFDATA='$SERVICEPERFDATA$' \
-o ACKAUTHOR='$SERVICEACKAUTHOR$' \
-o ACKCOMMENT='$SERVICEACKCOMMENT$' \
-o DURATION='$SERVICEDURATION$' \
-o SERVICEEVENTID='$SERVICEEVENTID$' \
-o LASTSERVICEEVENTID='$LASTSERVICEEVENTID$'
}