Merge pull request #237 from grravity/patch-2

Added headers, and new resources

docs/prerequisite-dns.md

@@ -8,6 +8,9 @@ Below you can find a list of **recommended DNS records**. While some are mandato
   ["Best Practices on Email Protection: SPF, DKIM and DMARC"](https://wiki.zimbra.com/wiki/Best_Practices_on_Email_Protection:_SPF,_DKIM_and_DMARC)
 - An in-depth discussion of SPF, DKIM and DMARC:
   ["How to eliminate spam and protect your name with DMARC"](https://www.skelleton.net/2015/03/21/how-to-eliminate-spam-and-protect-your-name-with-dmarc/)
+- A thorough guide on understanding DMARC:
+["Demystifying DMARC: A guide to preventing email spoofing"](https://seanthegeek.net/459/demystifying-dmarc/)
+
 
 ## Reverse DNS of your IP address
 
@@ -31,18 +34,21 @@ autoconfig          IN CNAME   mail
 In the example DNS zone file snippet below, a simple **SPF** TXT record is used to only allow THIS server (the MX) to send mail for your domain. Every other server is disallowed but able to ("`~all`"). Please refer to [SPF Project](http://www.open-spf.org/) for further reading.
 
 ```
+# Name              Type       Value
 @                   IN TXT     "v=spf1 mx -all"
 ```
 
 It is highly recommended to create a **DKIM** TXT record in your mailcow UI and set the corresponding TXT record in your DNS records. Please refer to [OpenDKIM](http://www.opendkim.org) for further reading.
 
 ```
+# Name              Type       Value
 dkim._domainkey     IN TXT     "v=DKIM1; k=rsa; t=s; s=email; p=..."
 ```
 
 The last step in protecting yourself and others is the implementation of a **DMARC** TXT record, for example by using the [DMARC Assistant](http://www.kitterman.com/dmarc/assistant.html) ([check](https://dmarcian.com/dmarc-inspector/google.com)).
 
 ```
+# Name              Type       Value
 _dmarc              IN TXT     "v=DMARC1; p=reject; rua=mailto:mailauth-reports@example.org"
 ```
 
@@ -51,6 +57,7 @@ _dmarc              IN TXT     "v=DMARC1; p=reject; rua=mailto:mailauth-reports@
 **SRV** records specify the server(s) for a specific protocol on your domain. If you want to explicitly announce a service as not provided, give "." as the target address (instead of "mail.example.org."). Please refer to [RFC 2782](https://tools.ietf.org/html/rfc2782).
 
 ```
+# Name              Type       Priority Weight Port    Value
 _imap._tcp          IN SRV     0        1      143      mail.example.org.
 _imaps._tcp         IN SRV     0        1      993      mail.example.org.
 _pop3._tcp          IN SRV     0        1      110      mail.example.org.
@@ -59,7 +66,7 @@ _submission._tcp    IN SRV     0 1 587   mail.example.org.
 _smtps._tcp         IN SRV     0        1      465      mail.example.org.
 _sieve._tcp         IN SRV     0        1      4190     mail.example.org.
 _autodiscover._tcp  IN SRV     0        1      443      mail.example.org.
-_carddavs._tcp      IN SRV     0 1 443   mail.example.org.
+_carddavs._tcp      IN SRV     0        1      443      Mail.example.org.
 _carddavs._tcp      IN TXT                              "path=/SOGo/dav/"
 _caldavs._tcp       IN SRV     0        1      443      mail.example.org.
 _caldavs._tcp       IN TXT                              "path=/SOGo/dav/"
@@ -73,13 +80,24 @@ Here are some tools you can use to verify your DNS configuration:
 - [port25.com](https://www.port25.com/dkim-wizard/) (DKIM, SPF)
 - [Mail-tester](https://www.mail-tester.com/) (DKIM, DMARC, SPF)
 - [DMARC Analyzer](https://www.dmarcanalyzer.com/spf/checker/) (DMARC, SPF)
+- [MultiRBL.valli.org](http://multirbl.valli.org/) (DNSBL, RBL, FCrDNS)
 
 ## Misc
 
-If you are interested in statistics, you can additionally register with the [Postmaster Tool](https://gmail.com/postmaster)  by Google and supply a **google-site-verification** TXT record, which will give you details about spam-classified mails by your domain. This is clearly optional.
+### Optional DMARC Statistics
+If you are interested in statistics, you can additionally register with some of the many below DMARC statistic services, or self-host your own.
 
-```
+**NOTE:** It is worth considering that if you request DMARC statistic reports to your mailcow server, if there are issues with that domain you may not get accurate results. You can consider using an alternative email domain for recieving DMARC reports.
-@                   IN TXT     "google-site-verification=..."
-```
 
+It is worth mentioning, that the following suggestions are not a comprehensive list of all services and tools avaialble, but only a small few of the many choices.
+
+- [Postmaster Tool](https://gmail.com/postmaster)
+- [parsedmarc](https://github.com/domainaware/parsedmarc) (self-hosted)
+- [Fraudmarc](https://fraudmarc.com/)
+- [Postmark](https://dmarc.postmarkapp.com)
+- [Dmarcian](https://dmarcian.com/)
+
+**NOTE:** The services may provide you with a TXT record, which you would insert into your DNS records as the provider specifies. This record will give you details about spam-classified mails by your domain. However, please ensure to read the providers documentation from the service you choose, as this process may vary and not all providers may use a TXT record.
+
+### Fully Qualified Domain Name (FQDN)
 [^1]: A **Fully Qualified Domain Name** (**FQDN**) is the complete (absolute) domain name for a specific computer or host, on the Internet. The FQDN consists of at least three parts divided by a dot: the hostname (myhost), the domain name (mydomain) and the top level domain in short **tld** (com). In the example of `mx.mailcow.email` the hostname would be `mx`, the domain name `mailcow` and the tld `email`.