Zimbra Collaboration Postscreen

Zimbra Collaboration Postscreen

   KB 22511        Last updated on 2023-09-11  

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Starting with Zimbra Collaboration 8.7 and above, Zimbra introduces Postscreen like an additional Anti-SPAM strategy. Zimbra Postscreen provides additional protection against mail server overload. One postscreen process handles multiple inbound SMTP connections, and decides which clients may talk to a Post-fix SMTP server process. By keeping spambots away, postscreen leaves more SMTP server processes available for legitimate clients, and delays the onset of server overload conditions.

Zimbra Collaboration Postscreen should not be used on SMTP ports that receive mail from end-user clients (MUAs). In a typical deployment, postscreen handles the MX service on TCP port 25, while MUA clients submit mail via the submission service on TCP port 587 which requires client authentication. Alternatively, a site could set up a dedicated, non-postscreen, "port 25" server that provides submission service and client authentication, but no MX service.

Zimbra Collaboration Postscreen maintains a temporary white-list for clients that have passed a number of tests. When an SMTP client IP address iswhitelisted, postscreen hands off the connection immediately to a Postfix SMTP server process. This minimizes the overhead for legitimate mail.

In a typical production setting, postscreen is configured to reject mail from clients that fail one or more tests. Zimbra Collaboration Postscreen logs rejected mail with the client address, helo, sender and recipient information.

Zimbra Collaboration Postscreen is not an SMTP proxy; this is intentional. The purpose is to keep spambots away from Postfix SMTP server processes, while minimizing overhead for legitimate traffic.

How it works

Scenario without Postscreen

A typical scenario without Postscreen, and without other Anti-SPAM security, will suffer of this common Problem, where bot and zombies talks with all the smtpd listeners that Zimbra is offering.

In this scenario, the good connections, or called other in this diagram, must wait until the bot or zombie finishes the communication, which sometimes can create a Timeout Error on Postfix for the good connections:

Mar 01 19:29:54 zimbrauk postfix/smtpd[24266]: timeout after RCPT from mail.example.com[]


Scenario with Postscreen

A typical scenario with Postscreen, where bot and zombies talks with Postscreen, who do all the basic checks, and who can deny the connection if the message is clearly from a bot or zombie, if the connection is not in the temporary whitelist, Postscreen will pass the Email to the local Anti-SPAM and Anti-Virus engines, who can accept it or deny it as usual. You can see how is the Mail Flow in Postscreen on the section below.

In this scenario, the good connections, or called other in this diagram, pass the Postscreen security and talks directly with the smtp daemon, who will scan the Email as usual with the AS/AV. All the bot or zombie are rejected by default.


Postscreen workflow

See attached the workflow for Zimbra Collaboration Postscreen


Zimbra attributes for Postscreen

Here you can find all the new attributes for Postscreen, and the link to the original Postfix description help per attribute.

Please note the difference between the ignore, enforce and drop for certain attributes:

  • ignore (default) - Ignore this result. Allow other tests to complete. Repeat this test the next time the client connects. This option is useful for testing and collecting statistics without blocking mail.
  • enforce - Allow other tests to complete. Reject attempts to deliver mail with a 550 SMTP reply, and log the helo/sender/recipient information. Repeat this test the next time the client connects.
  • drop - Drop the connection immediately with a 521 SMTP reply. Repeat this test the next time the client connects.
Name Description Type Optional in Default value Options
zimbraMtaPostscreenAccessList Value for postconf postscreen_access_list. Single valued, commas,separated list. string server,globalConfig permit_mynetworks
zimbraMtaPostscreenBareNewlineAction Value for postconf postscreen_bare_newline_action. enum server,globalConfig ignore ignore,enforce,drop
zimbraMtaPostscreenBareNewlineEnable Value for postconf postscreen_bare_newline_enable. enum server,globalConfig no yes,no
zimbraMtaPostscreenBareNewlineTTL Value for postconf postscreen_bare_newline_ttl. string server,globalConfig 30d
zimbraMtaPostscreenBlacklistAction Value for postconf postscreen_blacklist_action. enum server,globalConfig ignore ignore,enforce,drop
zimbraMtaPostscreenCacheCleanupInterval Value for postconf postscreen_cache_cleanup_interval. string server,globalConfig 12h
zimbraMtaPostscreenCacheRetentionTime Value for postconf postscreen_cache_retention_time. string server,globalConfig 7d
zimbraMtaPostscreenCommandCountLimit Value for postconf postscreen_command_count_limit. integer server,globalConfig 20
zimbraMtaPostscreenDnsblAction Value for postconf postscreen_dnsbl_action. enum server,globalConfig ignore ignore,enforce,drop
zimbraMtaPostscreenDnsblSites Value for postconf postscreen_dnsbl_sites. Multi valued, one DNSBL,value pair per attribute value. string server,globalConfig
zimbraMtaPostscreenDnsblThreshold Value for postconf postscreen_dnsbl_threshold. integer server,globalConfig 1
zimbraMtaPostscreenDnsblTTL Value for postconf postscreen_dnsbl_ttl. string server,globalConfig 1h
zimbraMtaPostscreenDnsblWhitelistThreshold Value for postconf postscreen_dnsbl_whitelist_threshold. integer server,globalConfig 0
zimbraMtaPostscreenGreetAction Value for postconf postscreen_greet_action. enum server,globalConfig ignore ignore,enforce,drop
zimbraMtaPostscreenGreetTTL Value for postconf postscreen_greet_ttl. string server,globalConfig 1d
zimbraMtaPostscreenNonSmtpCommandAction Value for postconf postscreen_non_smtp_command_action. enum server,globalConfig drop ignore,enforce,drop
zimbraMtaPostscreenNonSmtpCommandEnable Value for postconf postscreen_non_smtp_command_enable. enum server,globalConfig no yes,no
zimbraMtaPostscreenNonSmtpCommandTTL Value for postconf postscreen_non_smtp_command_ttl. string server,globalConfig 30d
zimbraMtaPostscreenPipeliningAction Value for postconf postscreen_pipelining_action. enum server,globalConfig enforce ignore,enforce,drop
zimbraMtaPostscreenPipeliningEnable Value for postconf postscreen_pipelining_enable. enum server,globalConfig no yes,no
zimbraMtaPostscreenPipeliningTTL Value for postconf postscreen_pipelining_ttl. string server,globalConfig 30d
zimbraMtaPostscreenWatchdogTimeout Value for postconf postscreen_watchdog_timeout. string server,globalConfig 10s
zimbraMtaPostscreenWhitelistInterfaces Value for postconf postscreen_whitelist_interfaces. Single valued,,comma separated list. string server,globalConfig static:all
zimbraMtaPostscreenDnsblMinTTL Value for postconf postscreen_dnsbl_min_ttl. tbd server,globalConfig tbd 60s
zimbraMtaPostscreenDnsblMaxTTL Value for postconf postscreen_dnsbl_max_ttl. tbd server,globalConfig tbd tbd
zimbraMtaPostscreenUpstreamProxyProtocol Value for postconf postscreen_upstream_proxy_protocol. Single valued, commas,separated list. enum server,globalConfig

How to enable it

Zimbra Collaboration Postscreen comes enabled by default in ZCS 8.7 or above, take a look to the previous Table where find all the defaults values per each Postscreen attribute.

Quick Example configuring Postscreen

Each scenario can be different, so please tune the next values according to your own Environment, in this case all values are set at GlobalConfig level: This configuration is medium/high level, enforcing a few attributes instead of ignore, change them to drop for higher level of security

zmprov mcf zimbraMtaPostscreenAccessList permit_mynetworks
zmprov mcf zimbraMtaPostscreenBareNewlineAction ignore
zmprov mcf zimbraMtaPostscreenBareNewlineEnable no
zmprov mcf zimbraMtaPostscreenBareNewlineTTL 30d
zmprov mcf zimbraMtaPostscreenBlacklistAction ignore
zmprov mcf zimbraMtaPostscreenCacheCleanupInterval 12h
zmprov mcf zimbraMtaPostscreenCacheRetentionTime 7d
zmprov mcf zimbraMtaPostscreenCommandCountLimit 20
zmprov mcf zimbraMtaPostscreenDnsblAction enforce
zmprov mcf zimbraMtaPostscreenDnsblSites 'b.barracudacentral.org=*7' zimbraMtaPostscreenDnsblSites 'your-dqs-key-here.zen.dq.spamhaus.net=127.0.0.[10;11]*8' zimbraMtaPostscreenDnsblSites 'your-dqs-key-here.zen.dq.spamhaus.net=127.0.0.[4..7]*6' zimbraMtaPostscreenDnsblSites 'your-dqs-key-here.zen.dq.spamhaus.net=*4' zimbraMtaPostscreenDnsblSites 'your-dqs-key-here.zen.dq.spamhaus.net=*3' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].0*-2' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].1*-3' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].2*-4' zimbraMtaPostscreenDnsblSites 'list.dnswl.org=127.0.[0..255].3*-5' zimbraMtaPostscreenDnsblSites 'bl.mailspike.net=*5' zimbraMtaPostscreenDnsblSites 'bl.mailspike.net=127.0.0.[10;11;12]*4' zimbraMtaPostscreenDnsblSites 'wl.mailspike.net=127.0.0.[18;19;20]*-2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*8' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*6' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*3' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*2' zimbraMtaPostscreenDnsblSites 'dnsbl.sorbs.net=*2'
zmprov mcf zimbraMtaPostscreenDnsblTTL 5m
zmprov mcf zimbraMtaPostscreenDnsblThreshold 8
zmprov mcf zimbraMtaPostscreenDnsblTimeout 10s
zmprov mcf zimbraMtaPostscreenDnsblWhitelistThreshold 0
zmprov mcf zimbraMtaPostscreenGreetAction enforce
zmprov mcf zimbraMtaPostscreenGreetTTL 1d
zmprov mcf zimbraMtaPostscreenNonSmtpCommandAction drop
zmprov mcf zimbraMtaPostscreenNonSmtpCommandEnable no
zmprov mcf zimbraMtaPostscreenNonSmtpCommandTTL 30d
zmprov mcf zimbraMtaPostscreenPipeliningAction enforce
zmprov mcf zimbraMtaPostscreenPipeliningEnable no
zmprov mcf zimbraMtaPostscreenPipeliningTTL 30d
zmprov mcf zimbraMtaPostscreenWatchdogTimeout 10s
zmprov mcf zimbraMtaPostscreenWhitelistInterfaces static:all
zmprov mcf zimbraMtaPostscreenDnsblReplyMap lmdb:/opt/zimbra/conf/dnsbl-reply

Get an account at Spamhaus and replace your-dqs-key-here with the DQS key of your Spamhaus account. Next you have to create a file /opt/zimbra/conf/dnsbl-reply with the following content:

'your-key-here'.sbl.dq.spamhaus.net     sbl.spamhaus.org
'your-key-here'.xbl.dq.spamhaus.net     xbl.spamhaus.org
'your-key-here'.pbl.dq.spamhaus.net     pbl.spamhaus.org
'your-key-here'.zen.dq.spamhaus.net     zen.spamhaus.org
'your-key-here'.dbl.dq.spamhaus.net     dbl.spamhaus.org
'your-key-here'.zrd.dq.spamhaus.net     zrd.spamhaus.org

Don't forget to run postmap /opt/zimbra/conf/dnsbl-reply command. Please note that the syntax for /opt/zimbra/conf/dnsbl-reply is not the same as/opt/zimbra/conf/dnsbl-reply-map used in https://wiki.zimbra.com/wiki/Anti-spam.

Finally as zimbra user restart the MTA by running

zmmtactl restart

For the latest configuration on Spamhaus go to your Spamhaus Portal click Products -> DQS in the top menu, then click the Manual tab and refer to section 3.1.2 Configuring Postfix.

Testing the Zimbra Collaboration Postscreen

Customers might want to set up the DNSBLs first, for example, but leave it on ignore. Postscreen will log what it would have done, but not do anything. Once you are satisfied it looks correct, then you can set values to enforce or drop in certain cases.

A real-world log example where you can see the error 550 from postscreen:

Mar  1 02:03:26 edge01 postfix/postscreen[23154]: DNSBL rank 28 for []:20438 
Mar  1 02:03:26 edge01 postfix/postscreen[23154]: CONNECT from []:58010 to []:25 
Mar  1 02:03:26 edge01 postfix/postscreen[23154]: WHITELISTED []:58010 
Mar  1 02:03:27 edge01 postfix/postscreen[23154]: NOQUEUE: reject: RCPT from []:20438: 550 5.7.1 Service unavailable; client [] blocked using zen.spamhaus.net; from=<hfxdgdsggfvfg@gmail.com>, to=<support@zimbra.com>, proto=ESMTP, helo=<gmail.com>
Mar  1 02:03:27 edge01 postfix/postscreen[23154]: DISCONNECT []:20438 

IP Whitelist and Blacklist using Postscreen

You can use now Postfix to whitelist or Blacklist IPs in an easier way by following the next steps:

  • Create /opt/zimbra/common/conf/postscreen_wblist
  • Add entries to it. I've only used it as a blacklist. The IP range should be on CIDR format:
# Rules are evaluated in the order as specified.
# Blacklist 60.70.80.* except permit reject reject
  • Set postscreen to use it:
zmprov mcf zimbraMtaPostscreenAccessList "permit_mynetworks, cidr:/opt/zimbra/common/conf/postscreen_wblist"
zmprov mcf zimbraMtaPostscreenBlacklistAction enforce
  • Wait for zmconfigd to pick up the change (60 seconds top)
  • After the 60 seconds, or a manual restart of the MTA services, you will see something like this on the Log:
Jun 29 05:16:22 edge04e postfix/postscreen[7546]: BLACKLISTED []:55699

Quick note on MTA Load Balancing, e.g. in Cloud Environments

For some load balancer systems, you may need to configure the setting below, as in the case where Postfix will see traffic from multiple load balancer nodes. In Amazon Web Services, if you provisioned an Elastic Network Load Balancer in the default recommended configuration (i.e., you did not turn on cross-Availability Zone load balancing), then you should NOT make the change below. This is because when you provision an AWS Network Load Balancer, AWS deploys one load balancer instance for each Availability Zone covered by the Network Load Balancer system. Consequently, each Zimbra MTA will see traffic from only one AWS Network Load Balancer node, and so you should NOT configure the setting below.

zmprov mcf zimbraMtaPostscreenUpstreamProxyProtocol haproxy

If your MTAs however will see traffic from multiple load balancer nodes, then make the change above, and verify the change is in progress:

tail -f /var/log/zimbra.log
Jun 24 17:24:29 zre-ldap004 zmconfigd[17944]: Fetching All configs
Jun 24 17:24:29 zre-ldap004 zmconfigd[17944]: All configs fetched in 0.08 seconds
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Watchdog: service antivirus status is OK.
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Var zimbraMtaPostscreenUpstreamProxyProtocol changed from 'None' -> 'haproxy'
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Rewrote: /opt/zimbra/common/conf/tag_as_originating.re with mode 440 (0.01 sec)
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Rewrote: /opt/zimbra/conf/postfix_header_checks with mode 440 (0.00 sec)
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Rewrote: /opt/zimbra/common/conf/tag_as_foreign.re with mode 440 (0.01 sec)
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Rewrote: /opt/zimbra/common/conf/master.cf with mode 440 (0.01 sec)
Jun 24 17:24:33 zre-ldap004 zmconfigd[17944]: Rewrote: /opt/zimbra/conf/mta_milter_options with mode 440 (0.00 sec)
Jun 24 17:24:36 zre-ldap004 zmconfigd[17944]: All rewrite threads completed in 2.93 sec
Jun 24 17:24:36 zre-ldap004 zmconfigd[17944]: controlProcess mta restart (-1)
Jun 24 17:24:36 zre-ldap004 zmconfigd[17944]: CONTROL mta: bin/zmmtactl reload norewrite
Jun 24 17:24:36 zre-ldap004 zmconfigd[17944]: mta reload initiated from zmconfigd
Jun 24 17:24:36 zre-ldap004 saslauthd[20153]: server_exit     : master exited: 20153
Jun 24 17:24:37 zre-ldap004 saslauthd[2925]: detach_tty      : master pid is: 2925
Jun 24 17:24:37 zre-ldap004 saslauthd[2925]: ipc_init        : listening on socket: /opt/zimbra/data/sasl2/state/mux
Jun 24 17:24:38 zre-ldap004 /postfix-script[2959]: refreshing the Postfix mail system
Jun 24 17:24:38 zre-ldap004 postfix/master[20304]: reload -- version 3.1.1, configuration /opt/zimbra/common/conf
Jun 24 17:24:38 zre-ldap004 zmconfigd[17944]: All restarts completed in 1.82 sec

And verify by running this command:

postconf postscreen_upstream_proxy_protocol
postscreen_upstream_proxy_protocol = haproxy

More information here

Additonal Content

Identified Support Issues

  • No Support issues reported yet.
Verified Against: Zimbra Collaboration Suite 8.7 Date Created: 01/03/2016
Article ID: https://wiki.zimbra.com/index.php?title=Zimbra_Collaboration_Postscreen Date Modified: 2023-09-11

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Wiki/KB reviewed by SME1 SME2 Copyeditor Last edit by Mark Stone
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