Error message - 5.0.0 smtp; 550 authentication required Permanent Failure - Other or Undefined Status

Hi, I ́m having trouble to recieve any massages from personal emails (seznam, gmail ets.). I can send email from my em client account, but when I send message from my personal email (xy@seznam.cz) to my em client account, message cannot be delivered and this error message is showed - Your message for from 2015/12/18 could not be delivered. It’s attached below. Důvod / Reason: --------------- 5.0.0 smtp; 550 authentication required Permanent Failure - Other or Undefined Status