What does “UGFzc3dvcmQ6″ mean?

mail 13:21:10 postfix/smtpd warning: unknown[5.135.19.156]: SASL LOGIN authentication failed: UGFzc3dvcmQ6

The answer is encoded in base64:

root# echo "UGFzc3dvcmQ6" | base64 -d
 Password:
 root#

So UGFzc3dvcmQ6 = Password:

When an IMAP client tries to connect to a Dovecot server using the SASL LOGIN protocol, Dovecot asks for the user’s password. If the SASL protocol attempted by the client doesn’t match the standard that Dovecot is expecting (common with spambots and hackers), Dovecot reports an error.
This gets reported to syslog as a postfix error, since Dovecot is launching Postfix to deliver the incoming e-mail message.