install ssmtp in Debian buster

I try to install ssmtp in Debian 10.0, but get the error Package ssmtp is not available, but is referred to by another package. This may mean that the package is missing, has been obsoleted, or is only available from another source But I get no result with neither apt search ssmpt nor: $ apt-file search ssmtp monitoring-plugins-basic: /usr/lib/nagios/plugins/check_ssmtp sosreport: /usr/share/sosreport/sos/plugins/ssmtp.py How do I install ssmtp in Debian buster?