I have been watching my mail queue and I keep seeing these errors in there.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

xxxxx@gmail.com
unrouteable mail domain "gmail.com"

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

xxxxx@hotmail.com
unrouteable mail domain "hotmail.com"

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

xxxxx@yeah.net
unrouteable mail domain "yeah.net"

Now the first two I know are working the other domains I have checked are working but for some reson I get this with a few email's. Anyone know why?

If you have cPanel/WHM

In your WHM go to Tweak settings and under Domains, select the checkbox to Prevent users from parking/adding on common internet domains. (ie hotmail.com, aol.com)

It could be that

1) Spamassasin has died
2) You went over your maximum number of emails per hour.