Configure reverse DNS
in order to avoid mail seen as spam. From command line you can check as follow
$ nslookup ktln2.org
Server: 127.0.0.1
Address: 127.0.0.1#53
Non-authoritative answer:
Name: ktln2.org
Address: 46.102.247.82
$ host 46.102.247.82
82.247.102.46.in-addr.arpa domain name pointer ktln2.org.
From your hosting you can look for a rDNS
button somewhere :)
In order to update the virtual database you have to execute the following command
# postmap /etc/postfix/virtual
Read this guide
to configure spamassassin
for debian.
It's also possible to move the spam to the apposite folder using this guide.