Added greylisting using postgrey (#495)

* Added greylisting using postgrey
* Updated the documentation

README.md

@@ -18,6 +18,7 @@ Includes:
 - opendmarc
 - fail2ban
 - fetchmail
+- postgrey
 - basic [sieve support](https://github.com/tomav/docker-mailserver/wiki/Configure-Sieve-filters) using dovecot
 - [LetsEncrypt](https://letsencrypt.org/) and self-signed certificates
 - persistent data and state (but think about backups!)
@@ -62,6 +63,7 @@ services:
     - ENABLE_SPAMASSASSIN=1
     - ENABLE_CLAMAV=1
     - ENABLE_FAIL2BAN=1
+    - ENABLE_POSTGREY=1
     - ONE_DIR=1
     - DMS_DEBUG=0
     cap_add:
@@ -211,6 +213,29 @@ Otherwise, `iptables` won't be able to ban IPs.
   - **empty** => postmaster@domain.com
   - => Specify the postmaster address
 
+#### ENABLE_POSTGREY
+
+  - **0** => `postgrey` is disabled
+  - 1 => `postgrey` is enabled
+
+##### POSTGREY_DELAY
+
+  - **300** => greylist for N seconds
+
+Note: This postgrey setting needs `ENABLE_POSTGREY=1`
+
+##### POSTGREY_MAX_AGE
+  
+  - **35** => delete entries older than N days since the last time that they have been seen
+
+Note: This postgrey setting needs `ENABLE_POSTGREY=1`
+
+##### POSTGREY_TEXT
+  
+  - **Delayed by postgrey** => response when a mail is greylisted
+
+Note: This postgrey setting needs `ENABLE_POSTGREY=1`
+
 ##### ENABLE_SASLAUTHD
 
   - **0** => `saslauthd` is disabled