I had a working Postfix server, but added a few lines to my main.cf in a hope to block some common spam. Those lines I added were: smtpd_helo_required = yes smtpd_recipient_restrictions = reject_invalid_hostname, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client multi.uribl.com, reject_rbl_client dsn.rfc-ignorant.org, reject_rbl_client dul.dnsbl.sorbs.net, reject_rbl_client list.dsbl.org, reject_rbl_client sbl-xbl.spamhaus.org, reject_rbl_client bl.spamcop.net, reject_rbl_client dnsbl.sorbs.net, reject_rbl_client cbl.abuseat.org, reject_rbl_client ix.dnsbl.manitu.net, reject_rbl_client combined.rbl.msrbl.net, reject_rbl_client rabl.nuclearelephant.com, permit It appears my postfix is now receiving normal emails fine, and blocking spam emails. But when I now try to use this server myself to send to a remote domain (an email not on my server) I get bounced, with maillog saying something like this: Nov 12 06:19:36 srv postfix/smtpd[11756]: NOQUEUE: reject: RCPT from unknown[xx.xx.x.xxx]: 450 4.1.2 <[email protected]>: Recipient address rejected: Domain not found; from=<[email protected]> to=<[email protected]> proto=ESMTP helo=<[192.168.1.100]> Is that saying 'domain not found' for gmail.com? Why is that recipient address rejected? An output of my postconf-n is: alias_database = hash:/etc/aliases alias_maps = hash:/etc/aliases broken_sasl_auth_clients = yes 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 mailbox_size_limit = 0 mailq_path = /usr/bin/mailq.postfix manpage_directory = /usr/share/man mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain mydomain = primarydomain.net myhostname = mail.primarydomain.net myorigin = $myhostname newaliases_path = /usr/bin/newaliases.postfix queue_directory = /var/spool/postfix readme_directory = /usr/share/doc/postfix-2.6.6/README_FILES relay_domains = $mydestination, primarydomain.net, secondarydomain.org sample_directory = /usr/share/doc/postfix-2.6.6/samples sendmail_path = /usr/sbin/sendmail.postfix setgid_group = postdrop smtpd_client_restrictions = permit_sasl_authenticated smtpd_helo_required = yes smtpd_recipient_restrictions = reject_invalid_hostname, reject_unknown_recipient_domain, reject_unauth_pipelining, permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, reject_rbl_client multi.uribl.com, reject_rbl_client dsn.rfc-ignorant.org, reject_rbl_client dul.dnsbl.sorbs.net, reject_rbl_client list.dsbl.org, reject_rbl_client sbl-xbl.spamhaus.org, reject_rbl_client bl.spamcop.net, reject_rbl_client dnsbl.sorbs.net, reject_rbl_client cbl.abuseat.org, reject_rbl_client ix.dnsbl.manitu.net, reject_rbl_client combined.rbl.msrbl.net, reject_rbl_client rabl.nuclearelephant.com, permit smtpd_sasl_auth_enable = yes smtpd_sasl_path = private/auth smtpd_sasl_type = dovecot smtpd_sender_restrictions = reject_unknown_sender_domain soft_bounce = no unknown_local_recipient_reject_code = 550 virtual_alias_domains = mail.secondarydomain.org virtual_alias_maps = hash:/etc/postfix/virtual Any insight greatly appreciated. Edit: here is the dig mx gmail.com from the server: ; <<>> DiG 9.8.2rc1-RedHat-9.8.2-0.17.rc1.el6_4.4 <<>> mx gmail.com ;; global options: +cmd ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 31766 ;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 4, ADDITIONAL: 14 ;; QUESTION SECTION: ;gmail.com. IN MX ;; ANSWER SECTION: gmail.com. 1207 IN MX 5 gmail-smtp-in.l.google.com. gmail.com. 1207 IN MX 30 alt3.gmail-smtp-in.l.google.com. gmail.com. 1207 IN MX 20 alt2.gmail-smtp-in.l.google.com. gmail.com. 1207 IN MX 40 alt4.gmail-smtp-in.l.google.com. gmail.com. 1207 IN MX 10 alt1.gmail-smtp-in.l.google.com. ;; AUTHORITY SECTION: gmail.com. 109168 IN NS ns1.google.com. gmail.com. 109168 IN NS ns4.google.com. gmail.com. 109168 IN NS ns3.google.com. gmail.com. 109168 IN NS ns2.google.com. ;; ADDITIONAL SECTION: alt1.gmail-smtp-in.l.google.com. 207 IN A 173.194.70.27 alt1.gmail-smtp-in.l.google.com. 248 IN AAAA 2a00:1450:4001:c02::1b gmail-smtp-in.l.google.com. 200 IN A 173.194.67.26 gmail-smtp-in.l.google.com. 248 IN AAAA 2a00:1450:400c:c05::1b alt3.gmail-smtp-in.l.google.com. 207 IN A 74.125.143.27 alt3.gmail-smtp-in.l.google.com. 249 IN AAAA 2a00:1450:400c:c05::1b alt2.gmail-smtp-in.l.google.com. 207 IN A 173.194.69.27 alt2.gmail-smtp-in.l.google.com. 248 IN AAAA 2a00:1450:4008:c01::1b alt4.gmail-smtp-in.l.google.com. 207 IN A 173.194.79.27 alt4.gmail-smtp-in.l.google.com. 249 IN AAAA 2607:f8b0:400e:c01::1a ns2.google.com. 281970 IN A 216.239.34.10 ns3.google.com. 281970 IN A 216.239.36.10 ns4.google.com. 281970 IN A 216.239.38.10 ns1.google.com. 281970 IN A 216.239.32.10