I have postfix v2.6.6 running on CentOS 6.3, hostname priest.ocsl.local (private, internal domain) with a number of aliases supportpeople: [email protected], [email protected], [email protected] requests: "|/opt/rt4/bin/rt-mailgate --queue 'general' --action correspond --url http://localhost/", supportpeople help: "|/opt/rt4/bin/rt-mailgate --queue 'help' --action correspond --url http://localhost/", supportpeople If I leave postfix with its default configuration, then the aliases are resolved correctly/as I expect, so that incoming mail to, say, [email protected] will be piped through the rt-mailgate mailgate command and also be delivered (via the mail server for ocsl.co.uk (a publicly resolvable domain)) to [email protected], user2, etc. The problem comes when I define mydomain = ocsl.co.uk in /etc/postfix/main.cf (with the intention that outgoing mail come from, for example, [email protected]). When I do this, postfix continues to run the piped command correctly, however it no longer expands the nested aliases as I expect: instead of trying to deliver to [email protected], user2 etc, it tries to send to [email protected], which does not exist on the upstream mail server and generates NDRs. postconf -n for the non-working configuration follows (the working configuration differs only by the "mydomain" line. alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases command_directory = /usr/sbin config_directory = /etc/postfix daemon_directory = /usr/libexec/postfix data_directory = /var/lib/postfix debug_peer_level = 2 html_directory = no inet_interfaces = all inet_protocols = all mail_owner = postfix mailq_path = /usr/bin/mailq.postfix manpage_directory = /usr/share/man mydestination = $myhostname, localhost.$mydomain, localhost mydomain = ocsl.co.uk newaliases_path = /usr/bin/newaliases.postfix queue_directory = /var/spool/postfix readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES sample_directory = /usr/share/doc/postfix-2.6.6/samples sendmail_path = /usr/sbin/sendmail.postfix setgid_group = postdrop unknown_local_recipient_reject_code = 550 We did have things working as we expected/wanted previously on an older system running Sendmail.