CiviMail Bounce Type - Syntax
Ideally this means, that there was an error in the smtp transaction: https://docs.civicrm.org/sysadmin/en/latest/setup/civimail/inbound/

However, this bounce type is also used for all bounces that could not be classified as one of the other types. Your best bet to actually find out what is going on is to look at the bounce reason in the bounce report or, if thats still not helpfull enough, to have a look into the actual mailbox.