SASL LOGIN authentication failed: UGFzc3dvcmQ6

hi,
if,
tail -f /var/log/maillog  file
SASL LOGIN authentication failed: UGFzc3dvcmQ6

fixed
nano /etc/dovecot/dovecot-mysql.conf

driver = mysql
connect = host=localhost dbname=postfix user=postfix password=xxxxxxxxxx
#default_pass_scheme = MD5-CRYPT
default_pass_scheme = PLAIN-MD5

remove md5-crypt 
add plain-md5
thats it...

 SASL LOGIN authentication failed is login attempts on your dovecot/postfix server from random person trying to attack your server.