Anti-spam Strategies

Revision as of 20:06, 28 July 2014 by Quanah (talk | contribs) (Postfix Tweaks)

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This article applies to the following ZCS versions.

ZCS 8.0 Article ZCS 8.0 ZCS 7.0 Article ZCS 7.0

For SpamAssassin and Anti-spam Updates

Customizing SpamAssassin

ZCS 8.5 and later

For ZCS 8.5, Spamassassin layout has been corrected as per the SpamAssassin developers. is migrated to the /opt/zimbra/data/spamassassin/localrules directory. This is the supported location for doing customizations of SpamAssassin for ZCS 8.5 and later.


For ZCS 8.0, SpamAssassin scans for all *.cf files in /opt/zimbra/conf/sa and loads them in alphabetical order. If you create a file, it will be loaded after is loaded. This is the supported method for doing customizations of SpamAssassin for ZCS8.0

In 8.0.5, two options for added to the product to enable SpamAssassin rule updates via sa-update [reference: see [1]]:

  • antispam_enable_rule_updates
  • antispam_enable_restarts

Check that these are set to true, and if not, set them to true and restart amavisd and the MTA:

$ zmlocalconfig -e antispam_enable_rule_updates
antispam_enable_rule_updates = false
$ zmlocalconfig antispam_enable_restarts
antispam_enable_restarts = false
$ zmlocalconfig -e antispam_enable_rule_updates=true
$ zmlocalconfig -e antispam_enable_restarts=true
$ zmamavisdctl restart
$ zmmtactl restart

ZCS 6 and ZCS7

For ZCS 6 and ZCS7, SpamAssassin customizations go in /opt/zimbra/conf/ When upgrading to ZCS8 the file will be reloacted to /opt/zimbra/conf/sa

Automatic rule updates

With ZCS 8 and later, it is possible to enable automatic rule updates for SpamAssassin to help improve scoring. There are two localconfig keys that control the automatic update behavior.

  • antispam_enable_rule_updates controls whether or not to enable automatic rule updates. Defaults to false.
  • antispam_enable_restarts controls whether or not Amavisd will be automatically restarted after a rule update if they are enabled. Defaults to false.

Automatic rule compilation

With ZCS 8.5 and later, it is possible to enable automatic rule compilation when automatic updates are enabled. Compiling the SA rules helps decrease the amount of time it takes to score email. This is controlled via a localconfig key.

  • antispam_enable_rule_compilation controls whether or not to automatically compile new rules that are automatically updated. Defaults to false.

Customizing Postfix

In ZCS 7 and ZCS 8, customizing Postfix is a mix of zmlocalconfig and zmprov settings. In ZCS 8.5, virutally all settings are done via zmprov (zmlocalconfig settings will be migrated on upgrade if they do not match the default value).

zmprov/zmlocalconfig are both permissible and the recommended way to perform Postfix customizations for supported keys.

For example:

zmprov ms <server> +zimbraMtaRestriction reject_unknown_reverse_client_hostname

Specific Suggested Tweaks

Last update 2 June 2014 by L. Mark Stone, Reliable Networks

Our client base is very nervous about spam-delivered malware but even more concerned about "false-positives" i.e. legitimate email incorrectly identified as spam. Consequently, we've had to develop tweaks to improve Zimbra's default SpamAssassin configurations. The results have been that users with very public email addresses who typically receive several hundred to more than a thousand emails per day will see no more than ~3 spam emails per day in their Inbox. In our experience, anything less than that and you are likely to wind up with false positives.

If your end-user base is more tolerant of false positives, then you can tighten things up.

Keep in mind that Zimbra's Postfix takes a cut at filtering the email stream before Zimbra's SpamAssassin, and that SpamAssassin's processing of emails is much more resource intensive than Postfix's. Consequently, any filtering that you can do at the Postfix level to block emails outright will be helpful in both blocking spam and lowering resource utilization on your Zimbra server. Just be careful of inducing false positives!

Postfix Tweaks

At the Postfix level we use just a few complementary and conservative RBLs, one DNS check and one Protocol check. All of these can be configured via the Admin Console: (Global Settings > MTA).

The RBLs we use are:


Additional RBLs used by zimbra are:

  • (Ignore this one if using, as it is already included in those results)

Untested RBLs:


The Client RHSBLs we use are (updated June 2, 2014):


Untested Client RHSBLs:


Adding RBL and RHSBLs checks in postfix can also be done via the command line.

For RBLs:

 zmprov mcf +zimbraMtaRestriction reject_rbl_client

For RHSBL clients:

 zmprov mcf +zimbraMtaRestriction reject_rhsbl_client

On the same Admin Console page we also enable (and leave the remaining Protocol and DNS checks disabled):

  • reject_non_fqdn_sender
  • reject_unknown_sender_domain (Note this setting will be updated in 8.0.5)

On that same page we also make sure disable "Add X-Originating-IP to messages" as this can block email from remote users with fat email clients like Outlook and Thunderbird on home and public networks like Internet cafes (ZWC clients are unaffected by this.)

SpamAssassin Tweaks via the Commandline

Our current recommended SpamAssassin customizations comprise three complementary methods:

  1. Increase the log level reported by Amavis to get clarity from SpamAssassin on why/how spam is being blocked and getting through.
  2. Put Amavis's temporary directory on a RAM disk to speed up processing.
  3. Tweak the scores for a few selected individual SpamAssassin tests after installing Pyzor and Razor2.
1. Increase Amavis's Log Level

We found that increasing the log level from 1 to 2 puts in /var/log/zimbra.log the specific SpamAssassin tests which each email has triggered.

Customizing the Amavis Loglevel is supported in ZCS 8.0.5 and later:

zmprov mcf zimbraAmavisLogLevel 2

If you are on an earlier release, this can be achieved by editing /opt/zimbra/conf/ You will need to change the file's permissions to be writable, edit the file, then change the permissions back. Probably a good idea to make a backup copy of the file first... The final edit should should look like this:

$log_level = 2; # verbosity 0..5 - 1 is the minimum for msg tracing

Restart amavis for the change to take effect (zmavavisdctl restart). If you are on ZCS 8.0.5 or later, zmconfigd will automatically restart Amavis for you if you change the loglevel.

Now when an email is marked as spam and an end user asks you "Why?", you can grep /opt/zimbra/log and find out exactly why. Note the sender and recipient email addresses in the actual log file snippet below have been altered for privacy:

Nov 26 13:55:02 mail2 amavis[19107]: (19107-13) SPAM, <> -> <>, Yes, score=17.071 tag=-10 tag2=3.8 kill=16 tests=[BAYES_99=4, HTML_MESSAGE=0.001, RAZOR2_CF_RANGE_51_100=0.5, RAZOR2_CF_RANGE_E8_51_100=1.886, RAZOR2_CHECK=2.75, RDNS_NONE=3.5, SPF_PASS=-0.001, T_HK_NAME_DR=0.01, URIBL_BLACK=2.725, URIBL_DBL_SPAM=1.7] autolearn=spam

ZCS8 logs:

Apr 21 13:55:54 edge01 amavis[32619]: (32619-05) spam-tag, <> -> <>, Yes, score=9.014 tagged_above=-10 required=3 tests=[BAYES_40=-0.001, DIGEST_MULTIPLE=0.293, DKIM_SIGNED=0.1, HTML_IMAGE_ONLY_32=0.001, HTML_IMAGE_RATIO_06=0.001, HTML_MESSAGE=0.001, MIME_HTML_ONLY=0.723, PYZOR_CHECK=2.75, RAZOR2_CF_RANGE_51_100=0.5, RAZOR2_CF_RANGE_E8_51_100=1.886, RAZOR2_CHECK=2.75, T_DKIM_INVALID=0.01] autolearn=no autolearn_force=no

In the above example you can see that the sending server has no PTR (Reverse DNS record) and has already been reported to Razor.

2. Put Amavis's Temp Dir on a RAM Disk

We have seen even with fast RAID10 arrays that Amavis's processing an email with large attachments through SpamAssassin can take as long as 10-20 seconds. Putting Amavis'd temp directory on a RAM disk cuts this down to 1-2 seconds. Ralf Hildebrandt's book on Postfix has a section describing how to size the RAM disk, and why this is entirely safe for mail flow even in the event of a server crash. After you've done the homework for sizing, all you need to do is:

  1. Stop amavis, mount the RAM disk, start amavis and then edit /etc/fstab to make the change permanent.

An /etc/fstab entry for a 1GB RAM disks on the server therefore looks like:

zimbra@host:~$ cat /etc/fstab | grep amavis
tmpfs /opt/zimbra/data/amavisd/tmp tmpfs defaults,noexec,nodev,nosuid,size=1024m,mode=750,uid=zimbra,gid=zimbra 0 0
3. Tweak Selected SpamAssasin Scores After Installing Pyzor and Razor2
Installing Razor and Pyzor on Ubuntu
  • aptitude install razor pyzor
Installing Razor and Pyzor on RHEL6/CentOS6
  • Create /etc/yum.repos.d/epel.repo

name=EPEL repository

  • yum update
  • yum install pyzor perl-Razor-Agent
Configuring Pyzor

As the zimbra user

pyzor --homedir /opt/zimbra/data/amavisd/.pyzor discover

Update /opt/zimbra/conf/sa/

# pyzor
use_pyzor 1
pyzor_path /usr/bin/pyzor
# DNS lookups for pyzor can time out easily.  Set the following line IF you want to give pyzor up to 20 seconds to respond
# may slow down email delivery
pyzor_timeout 20

Configuring Razor

As the zimbra user

razor-admin -home=/opt/zimbra/data/amavisd/.razor -create
razor-admin -home=/opt/zimbra/data/amavisd/.razor -discover
razor-admin -home=/opt/zimbra/data/amavisd/.razor -register -user

Update /opt/zimbra/conf/sa/

# razor
use_razor2 1

Update SpamAssassin scoring

After installing Pyzor and Razor2 and restarting Zimbra's Amavis to make sure these modules are loaded by SpamAssassin, we add custom (higher) scoring for certain SpamAssassin tests to the appropriate custom SpamAssassin configuration file, which on ZCS 8 should be /opt/zimbra/conf/sa/ Our complete now looks like this (as of June 2, 2014):

# Reliable Networks custom
# Last updated June 2, 2014
pyzor_options --homedir /opt/zimbra/amavisd/.pyzor
pyzor_timeout 10
use_razor2 1
use_pyzor 1
score URIBL_BLACK 3.250
score RAZOR2_CHECK 3.250
score PYZOR_CHECK 3.250
score BAYES_99 4.000
score BAYES_60 2.250
score BAYES_50 1.500
score BAYES_00 -0.500
score RDNS_NONE 3.500

Then run zmamavisdctl restart to load the new scores.

4. Add custom rules from Kevin McGrail to your scores (UNTESTED)
  • As zimbra user: cd /opt/zimbra/conf/sa

wget -O

There are reports of high false positives using these rules, so user beware

5. Add the SOUGHT ruleset to your scores (UNTESTED)

TODO: Document configuring for Zimbra

6. Enable DCC

The source for DCC can be obtained from Please read the restrictions and limitations carefully.

After downloading and extracting the source, as the zimbra user, you will need to build it. It will take several tools (gcc, make, wget, etc).

There is some setup to be done as root initially. This is assuming using version 1.3.154 of dcc, adjust as necessary:

 # mkdir -p /opt/zimbra/dcc-1.3.154
 # chown zimbra:zimbra /opt/zimbra/dcc-1.3.154
 # cd /opt/zimbra;ln -s dcc-1.3.154 dcc

Now, as zimbra we need to build the software. Here's an example of downloading, extracting, and building:

 [zimbra@host ~]$ cd /tmp
 [zimbra@host tmp]$ mkdir dcc
 [zimbra@host dcc]$ wget
 [zimbra@host dcc]$ tar xfz dcc.tar.Z
 [zimbra@host dcc]$ cd dcc-1.3.154
 [zimbra@host dcc-1.3.154]$ ./configure --homedir=/opt/zimbra/dcc-1.3.154 \
 --disable-sys-inst --with-uid=zimbra --disable-server \
 --disable-dccifd --disable-dccm \
 --with-updatedcc_pfile=/opt/zimbra/data/dcc \
 --with-rundir=/opt/zimbra/data/dcc/run \
 [zimbra@host dcc-1.3.154]$ make
 [zimbra@host dcc-1.3.154]$ make install
 [zimbra@host dcc-1.3.154]$ cd /opt/zimbra/data
 [zimbra@host data]$ mkdir -p dcc/run

As the zimbra user, update as appropriate for your Zimbra version:

 use_dcc 1
 dcc_path /opt/zimbra/dcc/bin/dccproc

For ZCS releases 8.0 and earlier, you will need to enable the dcc module by modifying the v310.pre file from SpamAssassin. Find the line that looks like:

#loadplugin Mail::SpamAssassin::Plugin::DCC

and uncomment it (remove the # sign)

Last, but not least, restart amavis to pick up the changes:

 [zimbra@host ~]$ zmamavisdctl restart

DNSWL registration

Register your MTAs with DNSWL:

Other notes

We have found that increasing the scores of the above selected SpamAssassin scores blocks a lot of spam that would otherwise get through.

As we make updates to our own configurations, we will endeavor to keep this page updated as well.

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