Issues related to setting up and using your mailserver and mail clients-Unrouteable address – This error is usually either dns error, or the daily email limit being reached (if you’ve enabled it)

1) Type:

dig MX domain.com address

for the domain that is “Unrouteable”.
This will check to see if it resolves.

2 If it does not, 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:

nameserver 127.0.0.1

to the top of the /etc/resolv.conf file to tell your system for using  your own local nameserver, if there other IPs are not working.

3) If the dig does work, then check the file:
/etc/virtual/limit

if there is a numerical number  has other than 0, then that limit has likely been reached.
Type:

cd /etc/virtual/usage/
ls -la

to check on the usage for each user.

you can change the limit of a higher number if you want.

4) If there you  lookup does work, but it’s not a limit issue, it may 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 resolves to in step 1).   Eg:

telnet 1.2.3.4 25

where 1.2.3.4 is the IP of  A record.. of the MX value from step 1.   If there you can not connect to it, then your ISP is blocking port 25 from your server out, and you would need to use this guide to force all email out through your ISPs mail system.