/etc/ssmtp/revaliases
# sSMTP aliases
#
# Format:       local_account:outgoing_address:mailhub
#
# Example: root:your_login@your.domain:mailhub.your.domain[:port]
# where [:port] is an optional port number that defaults to 25.
root:ansgar-l@gmx.de:mail.gmx.net:465
ansgar:ansgar-l@gmx.de:mail.gmx.net:465

/etc/ssmtp/ssmtp.conf
root=ansgar-l@gmx.de
mailhub=mail.gmx.net:465
rewriteDomain=gmx.net
hostname=gmx.net
UseTLS=YES
AuthUser=ansgar-l@gmx.de
AuthPass=[...]
FromLineOverride=NO