SMTP Socket Error or failed to set sender error....Recipient address rejected: Domain not found; CiviMail keeps trying
I was just encountering the same problem ...

There is already an issue on github: https://github.com/civicrm/org.civicrm.flexmailer/issues/23

It would be really helpful to get that solved. Currently, as workaround, I am switching to PHP mail(), which I expect to cause problems regarding spam reputation...