Setup GMX email with SSMTP

I setup the simplest outgoing email server possible with ssmtp. It is giving me the error Authorization failed (535 Authentication credentials invalid). Here is my configuration without comments:

# /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

Running output:

~$ 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)

I understand that this implies a wrong username or password, but I have confirmed the password by copying it directly from my password manager. I also tried setting my user name to only myemail.

Anyone have luck setting up a GMX account?
If the problem isn't username/password, what could it possibly be?