This error is usually either a dns error or the daily email limit being reached (if you've enabled it).
dig MX domain.com
for the domain that is "Unroutable".
It will let you know if the domain has any MX records set. 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 number other than 0, then that limit has likely been reached.
cd /etc/virtual/usage/ ls -la
to check the usage for each user.
You can change the limit to a higher number if you want.
If the lookup does work and it's not a limit issue, it might be that 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 first step. Eg:
telnet 18.104.22.168 25
where 22.214.171.124 is the IP of the A record of the MX value from the first step. If you cannot connect to it, then your ISP is blocking port 25 from your server outbound, and you'll need to use this guide to force all email out through your ISPs mail system.
Check if I have an open relay
You can check if you have an open relay at CheckOR.com.
You should almost never have any domains listed in the
/etc/virtual/whitelist_domains file as any email with any from address as any domain in that file can relay through your server. Use it as a last resort only. Be especially careful to NOT have commonly forged domains, such as hotmail, gmail, etc., in that file.