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