Auth error when sending mail through Postfix with SSMTP (local server)

Question

I'm trying to send mail from a banana pi running SSMTP to a raspberry pi running dovecot/postfix.

I used Thunderbird to login to my mail server it showed three times saying "Unable to log in at server. Probably wrong configuration, username or password." but somehow it worked after another try (for a second account to test it did not work). If I try to send a test email with echo "Hello World" | mail -s "test" [email protected] I get following error: sendmail: Server didn't like our AUTH LOGIN (535 5.7.8 Error: authentication failed: Invalid authentication mechanism)

The password is simple 12345 just to test it so I have no idea whats going wrong. First time I am trying to set up something like this. Every help would be awesome If it possible I would even turn off any authentication because it's just a project to test.

Here is my ssmtp.conf:

# Config file for sSMTP sendmail
#
# The person who gets all mail for userids < 1000
# Make this empty to disable rewriting.
[email protected]

# The place where the mail goes. The actual machine name is required no
# MX records are consulted. Commonly mailhosts are named mail.domain.com
mailhub=mail.local:25

# Where will the mail seem to come from?
rewriteDomain=mail.local

# The full hostname
[email protected]
#UseSTARTTKS=YES
#USETLS0YES
UseSTARTTLS=YES
[email protected]
AuthPass=12345
FromLineOverride=YES
# Are users allowed to set their own From: address?
# YES - Allow the user to specify their own From: address
# NO - Use the system generated From: address

Postfix main.cf:

# See /usr/share/postfix/main.cf.dist for a commented, more complete version


# Debian specific:  Specifying a file name will cause the first
# line of that file to be used as the name.  The Debian default
# is /etc/mailname.
#myorigin = /etc/mailname

smtpd_banner = $myhostname ESMTP $mail_name (Raspbian)
biff = no

# appending .domain is the MUA's job.
append_dot_mydomain = no

# Uncomment the next line to generate "delayed mail" warnings
#delay_warning_time = 4h

readme_directory = no

config in Dovecot:

  GNU nano 5.4     /etc/dovecot/conf.d/01-mail-stack-delivery.conf              
# Some general options
protocols = imap pop3 sieve
disable_plaintext_auth = yes
ssl = yes
ssl_cert = </etc/ssl/certs/ssl-cert-snakeoil.pem
ssl_key = </etc/ssl/private/ssl-cert-snakeoil.key
ssl_cipher_list = ALL:!LOW:!SSLv2:ALL:!aNULL:!ADH:!eNULL:!EXP:RC4+RSA:+HIGH:+ME>
mail_location = maildir:~/Maildir
auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234>

# IMAP configuration
protocol imap {
        mail_max_userip_connections = 10
        imap_client_workarounds = delay-newmail
}

# POP3 configuration
protocol pop3 {
        mail_max_userip_connections = 10
        pop3_client_workarounds = outlook-no-nuls oe-ns-eoh