Initial more details page to redirect from README on the ports section

docs/content/config/security/understanding-the-ports.md

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+## Overview of email ports
+
+### Explicit TLS (aka Opportunistic TLS) - Opt-in Encryption
+
+Communication on these ports begin in [cleartext](https://www.denimgroup.com/resources/blog/2007/10/cleartext-vs-pl/), indicating support for `STARTTLS`. If both client and server support `STARTTLS` the connection will be secured over TLS, otherwise no encryption will be used.
+
+Support for `STARTTLS` is not always implemented correctly, which can lead to leaking credentials(client sending too early) prior to a TLS connection being established. Third-parties such as some ISPs have also been known to intercept the `STARTTLS` exchange, modifying network traffic to prevent establishing a secure connection.
+
+Due to these security concerns, [RFC 8314 (Section 4.1)](https://tools.ietf.org/html/rfc8314#section-4.1) encourages you to **prefer Implicit TLS ports where possible**. 
+
+### Implicit TLS - Enforced Encryption
+
+Communication is always encrypted, avoiding the above mentioned issues with Explicit TLS.
+
+You may know of these ports as **SMTPS, POP3S, IMAPS**, which indicate the protocol in combination with a TLS connection. However, Explicit TLS ports provide the same benefit when `STARTTLS` is successfully negotiated; Implicit TLS better communicates the improved security to all three protocols (SMTP/POP3/IMAP over Implicit TLS). 
+
+Additionally, referring to port 465 as *SMTPS* would be incorrect, as it is a submissions port requiring authentication to proceed via *ESMTP*, whereas ESMTPS has a different meaning(STARTTLS supported). Port 25 may lack Implicit TLS, but can be configured to be more secure between trusted parties via MTA-STS, STARTTLS Policy List, DNSSEC and DANE.
+
+| Protocol | Explicit TLS<sup>1</sup> | Implicit TLS    | Purpose              |
+|----------|--------------------------|-----------------|----------------------|
+| SMTP     | 25                       | N/A             | Transfer<sup>2</sup> |
+| ESMTP    | 587                      | 465<sup>3</sup> | Submission           |
+| POP3     | 110                      | 995             | Retrieval            |
+| IMAP4    | 143                      | 993             | Retrieval            |
+
+1. A connection *may* be secured over TLS when both ends support `STARTTLS`. On ports 110, 143 and 587, `docker-mailserver` will reject a connection that cannot be secured. Port 25 is [required](https://serverfault.com/questions/623692/is-it-still-wrong-to-require-starttls-on-incoming-smtp-messages) to support insecure connections.
+2. Receives email, `docker-mailserver` additionally filters for spam and viruses. For submitting email to the server to be sent to third-parties, you should prefer the *submission* ports(465, 587) - which require authentication. Unless a relay host is configured(eg SendGrid), outgoing email will leave the server via port 25(thus outbound traffic must not be blocked by your provider or firewall).
+3. A *submission* port since 2018 ([RFC 8314](https://tools.ietf.org/html/rfc8314)). Previously a secure variant of port 25.
+
+## Security
+
+**TODO:** *This section should provide any related configuration advice, and probably expand on and link to resources about DANE, DNSSEC, MTA-STS and STARTTLS Policy list, with advice on how to configure/setup these added security layers.*
+
+**TODO:** *A related section or page on ciphers used may be useful, although less important for users to be concerned about.*
+
+### TLS connections on mail servers, compared to web browsers
+
+Unlike with HTTP where a web browser client communicates directly with the server providing a website, a secure TLS connection as discussed below is not the equivalent safety that HTTPS provides when the transit of email (receiving or sending) is sent through third-parties, as the secure connection is only between two machines, any additional machines (MTAs) between the MUA and the MDA depends on them establishing secure connections between one another successfully. 
+
+Other machines that facilitate a connection that generally aren't taken into account can exist between a client and server, such as those where your connection passes through your ISP provider are capable of compromising a cleartext connection through interception.