Added DNS chapter

docs/dns.md

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+Below you can find a list of **recommended DNS records**. While some are mandatory for a mail server (A, MX), others are recommended to build a good reputation score (TXT/SPF) or used for auto-configuration of mail clients (SRV).
+
+## References
+* A good article covering all relevant topics:
+  ["3 DNS Records Every Email Marketer Must Know"](https://www.rackaid.com/blog/email-dns-records)      
+* Another great one, but Zimbra as an example platform:
+  ["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/)
+
+## Reverse DNS of your IP
+
+Make sure that the PTR record of your IP matches the FQDN hostname of your mailcow host: `mail.domain.tld`. This record is usually set at the provider you leased the IP (server) from.
+
+## The minimal DNS configuration
+
+This example shows you a set of records for one domain. Each domain that is added to mailcow needs at least this set or records.
+
+```
+mail                IN A       1.2.3.4
+autodiscover        IN A       1.2.3.4
+autoconfig          IN A       1.2.3.4
+
+@                   IN MX 10   mail
+```
+
+## DKIM, SPF and DMARC
+
+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.openspf.org).
+
+```
+@                   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).
+
+```
+default._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)).
+
+```
+_dmarc              IN TXT     "v=DMARC1; p=reject; rua=mailto:mailauth-reports@example.org"
+```
+
+## The advanced DNS configuration
+
+**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.tld."). 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.
+_autoconfig._tcp    IN SRV     0 1 443 autoconfig.example.org.
+_autodiscover._tcp  IN SRV     0 1 443 autodiscover.example.org.
+```
+
+
+## Testing
+Here are some tools you can use to verify your DNS configuration:
+- [MX Toolbox](https://mxtoolbox.com/SuperTool.aspx) (DNS, SMTP, RBL)
+- [port25.com](https://www.port25.com/dkim-wizard/) (DKIM, SPF)
+- [HAD Pilot](https://www.had-pilot.com/testdetails.html) (DKIM, DMARC, SPF)
+- [DMARC Analyzer](https://www.dmarcanalyzer.com/spf-record-check/) (DMARC, SPF)
+
+## 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.
+
+```
+@                   IN TXT     "google-site-verification=..."
+```