We just upgraded to version 15.4.615 from 15.3.x and we suddenly can't send email.
Error : No MX records were found for the ."our"... domain. Status 544 5.4.4. Host not found.
Also suddenly there is bunch of emails in spool.
We receive emails from other than our hosted domains.
We haven't change any settings just upgraded to the latest 15.4.615 version