This error is usually either dns error, or the daily email limit being reached (if you've enabled it)
dig MX domain.com
for the domain that is "Unrouteable".
This will check to see if it resolves.
If it doesn't, then check your /etc/resolv.conf file to see which IPs are listed there for your nameserver lookups.
Since you likely run your own name server, you can add:
to the top of the /etc/resolv.conf file to tell your system to use your own local nameserver, if the other IPs are not working.
If the dig does work, then check the file:
if there is a numerical number other than 0, then that limit has likely been reached.
to check on the usage for each user.
you can change the limit to a higher number if you want.
If the lookup does work, but it's not a limit issue, it might be the ISP blocking the outbound port 25. The way to test that is to try to telnet to port 25 on the IP that the MX value resolved to in step 1)
telnet 188.8.131.52 25
is the IP of the A record.. of the MX value from step 1. If you cannot connect to it, then your ISP is blocking port 25 from your server out, and you'd need to use this guide
to force all email out through your ISPs mail system.