Added headers, and new resources

Added headers to the advanced dns configuration table as well as dkim spft and dmarc sections, for better readability and clarity, as well as to match the top portion which has headers.
Also added MultiRBL.valli.org as a resource for testing DNSBL, RBL, and FCrDNS.
And lastly added Postmark as an alternative suggestion for gmail's postmaster under misc.

docs/prerequisite-dns.md

@@ -31,18 +31,21 @@ 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 a -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"
 ```
 
@@ -51,18 +54,19 @@ _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).
 
 ```
-_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.
-_pop3s._tcp         IN SRV     0 1 995   mail.example.org.
-_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 TXT     "path=/SOGo/dav/"
-_caldavs._tcp       IN SRV     0 1 443   mail.example.org.
-_caldavs._tcp       IN TXT     "path=/SOGo/dav/"
+# 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.
+_pop3s._tcp         IN SRV     0        1      995      mail.example.org.
+_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 TXT                              "path=/SOGo/dav/"
+_caldavs._tcp       IN SRV     0        1      443      mail.example.org.
+_caldavs._tcp       IN TXT                              "path=/SOGo/dav/"
 ```
 
 ## Testing
@@ -73,10 +77,11 @@ 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.
+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. Another alternative service is [Postmark](https://dmarc.postmarkapp.com) These are clearly optional.
 
 ```
 @                   IN TXT     "google-site-verification=..."