Could see that this thread has been dead for a few months and no solution listed.
I have the same problem as described.
I am running ISPConfig 3 in a multiserver setup.
A bit on my setup :
web server : IP 192.168.90
mail server : IP 192.168.60 (postfix / dovecot)
db server : IP 192.168.40
These are behind a router with IP 18.104.22.168
ns1 and ns2 are on 22.214.171.124 / 67 (bind9)
All servers are controlled and hosted by me.
Ports opened on the router : 25, 110, 587 and 143 to IP 192.168.60
My mysql-virtual_relaydomains.cf :
user = ispconfig
password = something
dbname = dbispconfig
table = mail_transport
select_field = domain
where_field = domain
additional_conditions = and active = 'y'
hosts = localhost
Should the last line be hosts = localhost ?