# /etc/ssmtp/ssmtp.conf
root=myemail@gmx.com
mailhub=mail.gmx.com:465
rewriteDomain=gmx.com
hostname=HP-Q101
rewriteDomain=gmx.com
FromLineOverride=YES
AuthUser=myemail@gmx.com
AuthPass=my@super&secret*pa$$word
UseTLS=YES

~$ ssmtp -vvv webmaster@example.com < msg
[<-] 220 gmx.com (mrgmxus001) Nemesis ESMTP Service ready
[->] EHLO ASUS-N53SM
[<-] 250 AUTH LOGIN PLAIN
[->] AUTH LOGIN
[<-] 334 VXNlcm5hbWU6
[->] c3VwZXZvMZExZGdteZ3jb33=
[<-] 334 ZZFzc3dvcmZ3
[<-] 535 Authentication credentials invalid
ssmtp: Authorization failed (535 Authentication credentials invalid)

# /etc/ssmtp/ssmtp.conf
root=youremail@gmx.com
mailhub=mail.gmx.com:587
rewriteDomain=gmx.com
hostname=gmx.com
FromLineOverride=YES
AuthUser=youremail@gmx.com
AuthPass=yourpassword
UseSTARTTLS=YES