Unrouteable email error

We suddenly had our RT installation return this error when people send email to it:

reason: 550 Unrouteable address

To my knowledge nothing has changed recently and a reboot did not fix this.

  • the email forwarding is correct
  • the entries in /etc/alias are correct
  • the MySQL DB is not too full
  • the HDD is not too full
  • the email server is working, it’s just not routing to RT correctly
  • this is happening for all of our queues

We are running RT 3.6.5 and Exim for our mail server. The Exim config gets the queue addresses from the RT DB.

Any advice on what I could check?