Aloha! Run out own ideas. Here is setup: Machine with ISPConfig with web service only (public). DNS name subdomain.domain.tld. Web hosts www.domain.tld and test.domain.tld. Mail for domain.tld is configured and working fine on another machine. MX records are working fine and I'm pretty sure it is not a DNS trouble. So far so good. But when machine tries to send out email (with php mail() function) all mails goes well except to email@example.com The reason for this is because in /etc/postfix/local-host-names above line #### MAKE MANUAL ENTRIES BELOW THIS LINE! #### is record domain.tld and therefor in mail log i can see: Nov 4 00:01:23 subdomain sendmail: lA3M1N5n013538: firstname.lastname@example.org, ctladdr=apache (48/48), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30522, relay=[127.0.0.1] [127.0.0.1], dsn=5.1.1, stat=User unknown Which is true - on this machine there are no such user. The question is - how to get rid of domain.tld in /etc/postfix/local-host-names In ISPConfig websites are configured with External Mailserver. No Co-domains. Just two websites there www.domain.tld and test.domain.tld. domain.tld web goes as ServerAlias to www.domain.tld. Also I've tried to modify manually /etc/postfix/local-host-names and after postfix restart everything works fine for a while ..till I make some changes in ISPConfig again Any help appreciated! Thanks.